No last commit when up-to-date

Get the last commit title from the upstream to the head, so that
no `last_commit` line will be shown when the branch is up to date
with the upstream.

tool/vcs.rb

@@ -402,7 +402,9 @@ class VCS
       modified = log[/^Date:\s+(.*)/, 1]
       branch = cmd_read_at(srcdir, [gitcmd + %W[symbolic-ref --short HEAD]])
       branch.chomp!
-      title = cmd_read_at(srcdir, [gitcmd + %W[log --format=%s -n1 FETCH_HEAD..HEAD]])
+      upstream = cmd_read_at(srcdir, [gitcmd + %W[branch --list --format=%(upstream:short) #${branch}]])
+      upstream.chomp!
+      title = cmd_read_at(srcdir, [gitcmd + %W[log --format=%s -n1 #{upstream}..HEAD]])
       title = nil if title.empty?
       [last, changed, modified, branch, title]
     end