attr.c::path_matches(): the basename is part of the pathname

The function takes two strings (pathname and basename) as if they
are independent strings, but in reality, the latter is always
pointing into a substring in the former.

Clarify this relationship by expressing the latter as an offset into
the former.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

attr.c

@@ -655,7 +655,7 @@ static void prepare_attr_stack(const char *path, int dirlen)
 }
 
 static int path_matches(const char *pathname, int pathlen,
-			const char *basename,
+			int basename_offset,
 			const struct pattern *pat,
 			const char *base, int baselen)
 {
@@ -667,8 +667,8 @@ static int path_matches(const char *pathname, int pathlen,
 		return 0;
 
 	if (pat->flags & EXC_FLAG_NODIR) {
-		return match_basename(basename,
-				      pathlen - (basename - pathname),
+		return match_basename(pathname + basename_offset,
+				      pathlen - basename_offset,
 				      pattern, prefix,
 				      pat->patternlen, pat->flags);
 	}
@@ -701,7 +701,7 @@ static int fill_one(const char *what, struct match_attr *a, int rem)
 	return rem;
 }
 
-static int fill(const char *path, int pathlen, const char *basename,
+static int fill(const char *path, int pathlen, int basename_offset,
 		struct attr_stack *stk, int rem)
 {
 	int i;
@@ -711,7 +711,7 @@ static int fill(const char *path, int pathlen, const char *basename,
 		struct match_attr *a = stk->attrs[i];
 		if (a->is_macro)
 			continue;
-		if (path_matches(path, pathlen, basename,
+		if (path_matches(path, pathlen, basename_offset,
 				 &a->u.pat, base, stk->originlen))
 			rem = fill_one("fill", a, rem);
 	}
@@ -750,7 +750,8 @@ static void collect_all_attrs(const char *path)
 {
 	struct attr_stack *stk;
 	int i, pathlen, rem, dirlen;
-	const char *basename, *cp, *last_slash = NULL;
+	const char *cp, *last_slash = NULL;
+	int basename_offset;
 
 	for (cp = path; *cp; cp++) {
 		if (*cp == '/' && cp[1])
@@ -758,10 +759,10 @@ static void collect_all_attrs(const char *path)
 	}
 	pathlen = cp - path;
 	if (last_slash) {
-		basename = last_slash + 1;
+		basename_offset = last_slash + 1 - path;
 		dirlen = last_slash - path;
 	} else {
-		basename = path;
+		basename_offset = 0;
 		dirlen = 0;
 	}
 
@@ -771,7 +772,7 @@ static void collect_all_attrs(const char *path)
 
 	rem = attr_nr;
 	for (stk = attr_stack; 0 < rem && stk; stk = stk->prev)
-		rem = fill(path, pathlen, basename, stk, rem);
+		rem = fill(path, pathlen, basename_offset, stk, rem);
 }
 
 int git_check_attr(const char *path, int num, struct git_attr_check *check)