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>

sha1_name.c

@@ -1115,7 +1115,7 @@ int get_sha1_with_context_1(const char *name, unsigned char *sha1,
 			if (new_filename)
 				filename = new_filename;
 			ret = get_tree_entry(tree_sha1, filename, sha1, &oc->mode);
-			if (only_to_die) {
+			if (ret && only_to_die) {
 				diagnose_invalid_sha1_path(prefix, filename,
 							   tree_sha1, object_name);
 				free(object_name);

t/t1506-rev-parse-diagnosis.sh

@@ -171,4 +171,15 @@ test_expect_success 'relative path when startup_info is NULL' '
 	grep "BUG: startup_info struct is not initialized." error
 '
 
+test_expect_success '<commit>:file correctly diagnosed after a pathname' '
+	test_must_fail git rev-parse file.txt HEAD:file.txt 1>actual 2>error &&
+	test_i18ngrep ! "exists on disk" error &&
+	test_i18ngrep "unknown revision or path not in the working tree" error &&
+	cat >expect <<-\EOF &&
+	file.txt
+	HEAD:file.txt
+	EOF
+	test_cmp expect actual
+'
+
 test_done