attr: fix off-by-one directory component length calculation

attr: fix off-by-one directory component length calculation

94bc671 (Add directory pattern matching to attributes - 2012-12-08)
uses find_basename() to calculate the length of directory part in
prepare_attr_stack. This function expects the directory without the
trailing slash (as "origin" field in match_attr struct is without the
trailing slash). find_basename() includes the trailing slash and
confuses push/pop algorithm.

Consider path = "abc/def" and the push down code:

	while (1) {
		len = strlen(attr_stack->origin);
		if (dirlen <= len)
			break;
		cp = memchr(path + len + 1, '/', dirlen - len - 1);
		if (!cp)
			cp = path + dirlen;

dirlen is 4, not 3, without this patch. So when attr_stack->origin is
"abc", it'll miss the exit condition because 4 <= 3 is wrong. It'll
then try to push "abc/" down the attr stack (because "cp" would be
NULL). So we have both "abc" and "abc/" in the stack.

Next time when "abc/ghi" is checked, "abc/" is popped out because of
the off-by-one dirlen, only to be pushed back in again by the above
code. This repeats for all files in the same directory. Which means
at least one failed open syscall per file, or more if .gitattributes
exists.

This is the perf result with 10 runs on git.git:

Test                                     94bc671^          94bc671                   HEAD
----------------------------------------------------------------------------------------------------------
7810.1: grep worktree, cheap regex       0.02(0.01+0.04)   0.05(0.03+0.05) +150.0%   0.02(0.01+0.04) +0.0%
7810.2: grep worktree, expensive regex   0.25(0.94+0.01)   0.26(0.94+0.02) +4.0%     0.25(0.93+0.02) +0.0%
7810.3: grep --cached, cheap regex       0.11(0.10+0.00)   0.12(0.10+0.02) +9.1%     0.10(0.10+0.00) -9.1%
7810.4: grep --cached, expensive regex   0.61(0.60+0.01)   0.62(0.61+0.01) +1.6%     0.61(0.60+0.00) +0.0%

Reported-by: Ross Lagerwall <rosslagerwall@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>