make-snapshot: no merge commits in ChangeLog

Parents commit hashs in logs of merge commits are abbreviated to
necessary length depending on the repositories.  Exclude merge
commits from ChangeLog to make it stable.

tool/lib/vcs.rb

@@ -623,7 +623,7 @@ class VCS
 
     def format_changelog(path, arg)
       env = {'TZ' => 'JST-9', 'LANG' => 'C', 'LC_ALL' => 'C'}
-      cmd = %W"#{COMMAND} log --format=medium --notes=commits --topo-order"
+      cmd = %W"#{COMMAND} log --format=medium --notes=commits --topo-order --no-merges"
       date = "--date=iso-local"
       unless system(env, *cmd, date, chdir: @srcdir, out: NullDevice, exception: false)
         date = "--date=iso"