sha1_name: do not trigger detailed diagnosis for file arguments

sha1_name: do not trigger detailed diagnosis for file arguments

diagnose_invalid_sha1_path() is meant to be called to diagnose a
misspelt <treeish>:<pathname> when <pathname> does not exist in
<treeish>.  However, the code may call it if <treeish>:<pathname> is
invalid (which triggers another call with only_to_die == 1), but for
another reason. This happens when calling e.g.

  git log existing-file HEAD:existing-file

because existing-file is a path and not a revision, the code
verifies that the arguments that follow to be paths.  This leads to
an incorrect message like "existing-file does not exist in HEAD",
even though the path exists in HEAD.

Check that the search for <pathname> in <treeish> fails before
triggering the diagnosis.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>