Reduce matz's work, let git do it instead

doc/ChangeLog-2016

@@ -1,6 +0,0 @@
-------------------------------------------------------------------------
-r57181 | matz | 2016-12-26 01:35:51 +0900 (Mon, 26 Dec 2016) | 2 lines
-
-version.h (RUBY_VERSION): 2.5.0 development has started.
-
-------------------------------------------------------------------------

doc/ChangeLog-2017

@@ -1,6 +0,0 @@
-------------------------------------------------------------------------
-r61474 | matz | 2017-12-25 23:05:59 +0900 (Mon, 25 Dec 2017) | 2 lines
-
-version.h (RUBY_VERSION): 2.6.0 development has started.
-
-------------------------------------------------------------------------

doc/ChangeLog-2018

@@ -1,8 +0,0 @@
-commit 3a0471faa0d383392ba05b3a6409b973b7b009d1
-Author: matz <matz@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
-Date:   Tue Dec 25 22:45:17 2018
-
-    version.h (RUBY_VERSION): 2.7.0 development has started.
-    
-    
-    git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66555 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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 @@ class VCS
       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