verify_filename(): handle backslashes in "wildcards are pathspecs" rule

Commit 28fcc0b71a (pathspec: avoid the need of "--" when wildcard is
used, 2015-05-02) allowed:

  git rev-parse '*.c'

without the double-dash. But the rule it uses to check for wildcards
actually looks for any glob special. This is overly liberal, as it means
that a pattern that doesn't actually do any wildcard matching, like
"a\b", will be considered a pathspec.

If you do have such a file on disk, that's presumably what you wanted.
But if you don't, the results are confusing: rather than say "there's no
such path a\b", we'll quietly accept it as a pathspec which very likely
matches nothing (or at least not what you intended). Likewise, looking
for path "a\*b" doesn't expand the search at all; it would only find a
single entry, "a*b".

This commit switches the rule to trigger only when glob metacharacters
would expand the search, meaning both of those cases will now report an
error (you can still disambiguate using "--", of course; we're just
tightening the DWIM heuristic).

Note that we didn't test the original feature in 28fcc0b71a at all. So
this patch not only tests for these corner cases, but also adds a
regression test for the existing behavior.

Reported-by: David Burström <davidburstrom@spotify.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -197,9 +197,26 @@ static void NORETURN die_verify_filename(struct repository *r,
  */
 static int looks_like_pathspec(const char *arg)
 {
-	/* anything with a wildcard character */
-	if (!no_wildcard(arg))
-		return 1;
+	const char *p;
+	int escaped = 0;
+
+	/*
+	 * Wildcard characters imply the user is looking to match pathspecs
+	 * that aren't in the filesystem. Note that this doesn't include
+	 * backslash even though it's a glob special; by itself it doesn't
+	 * cause any increase in the match. Likewise ignore backslash-escaped
+	 * wildcard characters.
+	 */
+	for (p = arg; *p; p++) {
+		if (escaped) {
+			escaped = 0;
+		} else if (is_glob_special(*p)) {
+			if (*p == '\\')
+				escaped = 1;
+			else
+				return 1;
+		}
+	}
 
 	/* long-form pathspec magic */
 	if (starts_with(arg, ":("))

t/t1506-rev-parse-diagnosis.sh

@@ -222,4 +222,18 @@ test_expect_success 'reject Nth ancestor if N is too high' '
 	test_must_fail git rev-parse HEAD~100000000000000000000000000000000
 '
 
+test_expect_success 'pathspecs with wildcards are not ambiguous' '
+	echo "*.c" >expect &&
+	git rev-parse "*.c" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'backslash does not trigger wildcard rule' '
+	test_must_fail git rev-parse "foo\\bar"
+'
+
+test_expect_success 'escaped char does not trigger wildcard rule' '
+	test_must_fail git rev-parse "foo\\*bar"
+'
+
 test_done