attr: allow pattern escape using backslash

.gitattributes pattern syntax is supposed to be the same as .gitignore (except a few things that do not make sense in attr context, but that's a different issue). .gitignore uses fnmatch() as the matching machinery and "\" is accepted as an escape code. In theory the pattern 'foo\ bar' should match path 'foo bar' in .gitignore. Granted, no one would write 'foo\ bar' in .gitignore when 'foo bar' should suffice.

Regardless, 'foo\ bar attr' does not (but should) attach "attr" to path "foo bar" because pattern/attr parse code does not understand backslash escape. It parses the line as path 'foo\' and attributes 'bar' and 'attr'. This patch teaches attr code to recognize the backslash in patterns (not macro names) and pass 'foo\ bar' down to fnmatch().

This changes the attr behavior. "foo\ attr", if exists in the field, would match nothing because path "foo\" is invalid in UNIX and is a directory in Windows, which we do not accept attaching attributes to. With this patch, that line becomes invalid and is rejected. So it's not really bad (i.e. no silent changes in behavior).

Other subtle behavioral changes may happen. Still, I think we should do this as it seems like a correct thing to do.

---
It was posted [1] during rc cycles (I think) and initial feedback from Junio was "not utterly wrong". I still think this is worth doing, hence this resend for discussion.

[1] http://thread.gmane.org/gmane.comp.version-control.git/207135

 attr.c                | 12 +++++++++++-
 t/t0003-attributes.sh |  5 +++++
 2 files changed, 16 insertions(+), 1 deletion(-)

index 887a9ae..173d51d 100644
@@ -221,8 +221,18 @@ static struct match_attr *parse_attr_line(const char *line, const char *src,
 			return NULL;
 		}
 	}
-	else
+	else {
 		is_macro = 0;
+		namelen = 0;
+		while (name[namelen] != '\0' &&
+		       !strchr(blank, name[namelen])) {
+			if (name[namelen] == '\\' &&
+			    name[namelen + 1] != '\0')
+				namelen += 2;
+			else
+				namelen++;
+		}
+	}
 
 	states = name + namelen;
 	states += strspn(states, blank);

index febc45c..16b419e 100755
@@ -24,6 +24,7 @@ test_expect_success 'setup' '
 		echo "offon -test test"
 		echo "no notest"
 		echo "A/e/F test=A/e/F"
+		echo "A\\ b test=space"
 	) >.gitattributes &&
 	(
 		echo "g test=a/g" &&

@@ -196,6 +197,10 @@ test_expect_success 'root subdir attribute test' '
 	attr_check subdir/a/i unspecified
 '
 
+test_expect_success 'quoting in pattern' '
+	attr_check "A b" space
+'
+
 test_expect_success 'setup bare' '
 	git clone --bare . bare.git &&
 	cd bare.git

1.7.12.1.406.g6ab07c4