Make !pattern in .gitattributes non-fatal

Before 82dce998 (attr: more matching optimizations from .gitignore,
2012-10-15), .gitattributes did not have any special treatment of a
leading '!'.  The docs, however, always said

  The rules how the pattern matches paths are the same as in
  `.gitignore` files; see linkgit:gitignore[5].

By those rules, leading '!' means pattern negation.  So 82dce998
correctly determined that this kind of line makes no sense and should
be disallowed.

However, users who actually had a rule for files starting with a '!'
are in a bad position: before 82dce998 '!' matched that literal
character, so it is conceivable that users have .gitattributes with
such lines in them.  After 82dce998 the unescaped version was
disallowed in such a way that git outright refuses to run(!) most
commands in the presence of such a .gitattributes.  It therefore
becomes very hard to fix, let alone work with, such repositories.

Let's at least allow the users to fix their repos: change the fatal
error into a warning.

Reported-by: mathstuf@gmail.com
Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

attr.c

@@ -255,9 +255,11 @@ static struct match_attr *parse_attr_line(const char *line, const char *src,
 				      &res->u.pat.patternlen,
 				      &res->u.pat.flags,
 				      &res->u.pat.nowildcardlen);
-		if (res->u.pat.flags & EXC_FLAG_NEGATIVE)
-			die(_("Negative patterns are forbidden in git attributes\n"
-			      "Use '\\!' for literal leading exclamation."));
+		if (res->u.pat.flags & EXC_FLAG_NEGATIVE) {
+			warning(_("Negative patterns are ignored in git attributes\n"
+				  "Use '\\!' for literal leading exclamation."));
+			return NULL;
+		}
 	}
 	res->is_macro = is_macro;
 	res->num_attr = num_attr;

t/t0003-attributes.sh

@@ -198,7 +198,8 @@ test_expect_success 'root subdir attribute test' '
 
 test_expect_success 'negative patterns' '
 	echo "!f test=bar" >.gitattributes &&
-	test_must_fail git check-attr test -- f
+	git check-attr test -- '"'"'!f'"'"' 2>errors &&
+	test_i18ngrep "Negative patterns are ignored" errors
 '
 
 test_expect_success 'patterns starting with exclamation' '