[BUG] 'git show' gives duplicate errors for ambiguous args

With the git.git repository:

  $ git show abcd
  error: short SHA1 abcd is ambiguous.
  error: short SHA1 abcd is ambiguous.
  fatal: ambiguous argument 'abcd': unknown revision or path not in the
working tree.
  Use '--' to separate paths from revisions

The "is ambiguous" message shouldn't be shown twice.

The first error is printed by revision.c:1796 (a call to
handle_revision_arg), and the second by revision.c:1808 (a call to
verify_filename). I don't see an easy way to suppress it.