Fix VCS::GIT#export_changelog

    + gmake RELNAME=master@0f1eea0 AUTOCONF=autoconf dist
    /home/chkbuild/.rbenv/versions/3.4.8/bin/ruby --disable=gems  ./tool/make-snapshot \
    -srcdir=. -packages=gzip,zip,all \
    -unicode-version=17.0.0 \
    tmp master@0f1eea09490
    Exporting master@0f1eea09490
    Cloning into '/home/chkbuild/chkbuild/tmp/build/20260121T093003Z/tmp/ruby-snapshot20260121-3800606-t2y44s/ruby'...
    done.
    fatal: bad revision '#<struct VCS::GIT::Branch to_str="master">'
    fatal: bad revision '#<struct VCS::GIT::Branch to_str="master">'
    ./tool/make-snapshot:399: warning: no starting commit found

Author: etiennebarrie

tool/lib/vcs.rb

@@ -459,7 +459,8 @@ def export_changelog(url = '@', from = nil, to = nil, _path = nil, path: _path,
         rev or next
         rev unless rev.empty?
       end
-      unless from&.match?(/./) or (from = branch_beginning(url))&.match?(/./)
+      to ||= url.to_str
+      unless from&.match?(/./) or (from = branch_beginning(to))&.match?(/./)
         warn "no starting commit found", uplevel: 1
         from = nil
       end
@@ -470,7 +471,6 @@ def export_changelog(url = '@', from = nil, to = nil, _path = nil, path: _path,
       else
         warn "Could not fetch notes/commits tree", uplevel: 1
       end
-      to ||= url.to_str
       if from
         arg = ["#{from}^..#{to}"]
       else