verify_filename(): ask the caller to chose the kind of diagnosis

verify_filename() can be called in two different contexts. Either we
just tried to interpret a string as an object name, and it fails, so
we try looking for a working tree file (i.e. we finished looking at
revs that come earlier on the command line, and the next argument
must be a pathname), or we _know_ that we are looking for a
pathname, and shouldn't even try interpreting the string as an
object name.

For example, with this change, we get:

  $ git log COPYING HEAD:inexistant
  fatal: HEAD:inexistant: no such path in the working tree.
  Use '-- <path>...' to specify paths that do not exist locally.
  $ git log HEAD:inexistant
  fatal: Path 'inexistant' does not exist in 'HEAD'

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

builtin/grep.c

@@ -1045,7 +1045,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (!seen_dashdash) {
 		int j;
 		for (j = i; j < argc; j++)
-			verify_filename(prefix, argv[j]);
+			verify_filename(prefix, argv[j], j == i);
 	}
 
 	paths = get_pathspec(prefix, argv + i);