get_sha1: avoid repeating ourselves via ONLY_TO_DIE

When the revision code cannot parse an argument like
"HEAD:foo", it will call maybe_die_on_misspelt_object_name(),
which re-runs get_sha1() with an extra ONLY_TO_DIE flag. We
then spend more effort to generate a better error message.

Unfortunately, a side effect is that our second call may
repeat the same error messages from the original get_sha1()
call. You can see this with:

  $ git show 0017
  error: short SHA1 0017 is ambiguous.
  error: short SHA1 0017 is ambiguous.
  fatal: ambiguous argument '0017': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

where the second "error:" line comes from the ONLY_TO_DIE
call.

To fix this, we can make ONLY_TO_DIE imply QUIETLY. This is
a little odd, because the whole point of ONLY_TO_DIE is to
output error messages. But what we want to do is tell the
rest of the get_sha1() code (particularly get_sha1_1()) that
the _regular_ messages should be quiet, but the only-to-die
ones should not.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -1386,6 +1386,9 @@ static int get_sha1_with_context_1(const char *name,
 	const char *cp;
 	int only_to_die = flags & GET_SHA1_ONLY_TO_DIE;
 
+	if (only_to_die)
+		flags |= GET_SHA1_QUIETLY;
+
 	memset(oc, 0, sizeof(*oc));
 	oc->mode = S_IFINVALID;
 	ret = get_sha1_1(name, namelen, sha1, flags);

t/t1512-rev-parse-disambiguation.sh

@@ -291,4 +291,10 @@ test_expect_success 'ambiguous short sha1 ref' '
 	grep "refname.*${REF}.*ambiguous" err
 '
 
+test_expect_success C_LOCALE_OUTPUT 'ambiguity errors are not repeated' '
+	test_must_fail git rev-parse 00000 2>stderr &&
+	grep "is ambiguous" stderr >errors &&
+	test_line_count = 1 errors
+'
+
 test_done