Reduce matz's work, let git do it instead

tool/make-snapshot

@@ -338,18 +338,11 @@ def package(vcs, rev, destdir, tmp = nil)
   def (clean = []).add(n) push(n); n end
   Dir.chdir(v) do
     unless File.exist?("ChangeLog")
-      # get last revision from previous ChangeLog archive
-      last_ChangeLog = Dir["doc/ChangeLog-*"].grep(/-(\d+)\z/) {|n| [$1.to_i, n]}.max[1]
-      open(last_ChangeLog) do |f|
-        if /\Acommit (\w+)/ =~ f.readline
-          beginning = $1
-        elsif /\Ar(\d+) / =~ f.readline
-          beginning = $1.to_i
-        else
-          abort "#{File.basename $0}: Cannot find revision from '#{last_ChangeLog}'"
-        end
-        vcs.export_changelog(url, beginning, revision, "ChangeLog")
+      # get the beginning revision from matz's commit
+      unless beginning = vcs.branch_beginning
+        abort "#{File.basename $0}: Cannot find revision from '#{last_ChangeLog}'"
       end
+      vcs.export_changelog(url, beginning, revision, "ChangeLog")
     end
 
     File.open(clean.add("cross.rb"), "w") do |f|

tool/vcs.rb

@@ -451,6 +451,12 @@ def after_export(dir)
       FileUtils.rm_rf(Dir.glob("#{dir}/.git*"))
     end
 
+    def branch_beginning
+      cmd_read(%W[ #{COMMAND} log -n1 --format=format:%H --reverse
+                   --author=matz --committer=matz --grep=start
+                   -- version.h include/ruby/version.h])
+    end
+
     def export_changelog(url, from, to, path)
       range = [from, to].map do |rev|
         rev or next