Defer VCS detection when too many jobs are running

In cases like using `:cfdo`, many buffers may be quickly visited, thus
starting lots of VCS detection jobs.  This can lead to the (n)vim
process having too many open files and all the detection jobs noisily
failing.

Limit VCS detection to 50 concurrent jobs to mitigate this problem.
Since `b:sy` is not being set when detection is deferred, the next visit
to the buffer will still run VCS detection.

Closes #333

Author: jamessan

autoload/sy.vim

@@ -10,6 +10,11 @@ function! sy#start(...) abort
     return
   endif
 
+  if g:signify_detecting > 50
+    call sy#verbose('Too many detection jobs running, deferring detection')
+    return
+  endif
+
   let bufnr = a:0 && has_key(a:1, 'bufnr') ? a:1.bufnr : bufnr('')
   let sy = getbufvar(bufnr, 'sy')
 

autoload/sy/repo.vim

@@ -7,6 +7,7 @@ function! sy#repo#detect(bufnr) abort
   let sy = getbufvar(a:bufnr, 'sy')
   for vcs in s:vcs_list
     let sy.detecting += 1
+    let g:signify_detecting += 1
     call sy#repo#get_diff(a:bufnr, vcs, function('sy#sign#set_signs'))
   endfor
 endfunction
@@ -130,6 +131,7 @@ function! s:handle_diff(options, exitval) abort
     call sy#verbose(printf('Signs already got updated by %s.', sy.updated_by), a:options.vcs)
     return
   elseif empty(sy.vcs)
+    let g:signify_detecting -= 1
     let sy.detecting -= 1
   endif
 

plugin/signify.vim

@@ -8,6 +8,7 @@ endif
 
 let g:loaded_signify = 1
 let g:signify_locked = 0
+let g:signify_detecting = 0
 
 " Commands {{{1
 command! -nargs=0 -bar       SignifyList            call sy#debug#list_active_buffers()