test-convert-git.t: make copy detection deterministic

Since both 'bar' and 'bar-copied' matched 'bar-copied2', Git's copy detection
would sometimes result in 'bar' being the source for 'bar-copied2' and
sometimes 'bar-copied'. Change bar in a separate commit so that that would no
longer be the case.

tests/test-convert-git.t

@@ -278,6 +278,8 b' input validation'
   R foo
 
   $ cd git-repo2
+  $ echo bar2 >> bar
+  $ commit -a -m 'change bar'
   $ cp bar bar-copied2
   $ git add bar-copied2
   $ commit -a -m 'copy with no changes'