Make FileUpdateChecker faster with many extensions

Projects using propshaft/importmap-rails can end up constructing FileUpdateCheckers with globs looking like:

"{/Users/jon/Developer/rails-importmaps/app/assets/images/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/app/assets/stylesheets/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/stimulus-rails-1.3.4/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/turbo-rails-2.0.11/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/actiontext-8.0.0/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/actiontext-8.0.0/app/assets/stylesheets/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/actioncable-8.0.0/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/activestorage-8.0.0/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/.devenv/bundle/ruby/3.3.0/gems/actionview-8.0.0/app/assets/javascripts/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/app/javascript/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream},
/Users/jon/Developer/rails-importmaps/vendor/javascript/**/*.{html,xhtml,text,txt,js,css,ics,csv,vcf,vtt,png,jpeg,jpg,jpe,pjpeg,gif,bmp,tiff,tif,svg,webp,mpeg,mpg,mpe,mp3,mp1,mp2,ogg,oga,spx,opus,m4a,mpg4,aac,webm,mp4,m4v,otf,ttf,woff,woff2,xml,rss,atom,yaml,yml,multipart_form,url_encoded_form,json,pdf,zip,gzip,gz,turbo_stream}}"

Generally speaking, globs with nested braces seem to perform very poorly.

If the glob looks like `app/assets/stylesheets/**/*.{css,scss,js}`, you just get one system call for each of app, app/assets, app/assets/stylesheets. This is the best-case scenario.
If the glob looks like `{app/assets/stylesheets/**/*.{css,scss,js}}` - which ought to be identical - you get three system calls for each of app, app/assets, app/assets/stylesheets. If you add more extensions to the inner glob, you multiply the number of system calls for every directory in the path.

Fortunately, we can avoid this performance drop by just passing the directories as separate arguments to `Dir[]` - ie change

`Dir["{foo/**/*.{css,js},bar/**/*.{css,js}}"]`

into

`Dir["foo/**/*.{css,js}", "bar/**/*.{css,js}"]`

On a fresh importmap-rails project on macos 15.1 with ruby 3.3.5, `Rails.application.assets.load_path.cache_sweeper.execute_if_updated` takes around 15ms. After this optimization, it's around 0.5ms.

Author: jdelStrother

activesupport/CHANGELOG.md

@@ -28,4 +28,8 @@
 
     *Taketo Takashima*
 
+*   Make `ActiveSupport::FileUpdateChecker` faster when checking many file-extensions.
+
+    *Jonathan del Strother*
+
 Please check [8-0-stable](https://github.com/rails/rails/blob/8-0-stable/activesupport/CHANGELOG.md) for previous changes.

activesupport/lib/active_support/file_update_checker.rb

@@ -47,7 +47,7 @@ def initialize(files, dirs = {}, &block)
       end
 
       @files = files.freeze
-      @glob  = compile_glob(dirs)
+      @globs = compile_glob(dirs)
       @block = block
 
       @watched    = nil
@@ -103,7 +103,7 @@ def execute_if_updated
       def watched
         @watched || begin
           all = @files.select { |f| File.exist?(f) }
-          all.concat(Dir[@glob]) if @glob
+          all.concat(Dir[*@globs]) if @globs
           all.tap(&:uniq!)
         end
       end
@@ -145,10 +145,9 @@ def compile_glob(hash)
         hash.freeze # Freeze so changes aren't accidentally pushed
         return if hash.empty?
 
-        globs = hash.map do |key, value|
+        hash.map do |key, value|
           "#{escape(key)}/**/*#{compile_ext(value)}"
         end
-        "{#{globs.join(",")}}"
       end
 
       def escape(key)