wildmatch: make a special case for "*/" with FNM_PATHNAME

Normally we need recursion for "*". In this case we know that it
matches everything until "/" so we can skip the recursion.

glibc, '*/*/*' on linux-2.6.git file list 2000 times
before:
wildmatch 8s 74513us
fnmatch   1s 97042us or 13.59% faster
after:
wildmatch 3s 521862us
fnmatch   3s 488616us or 99.06% slower

Same test with compat/fnmatch:
wildmatch 8s 110763us
fnmatch   2s 980845us or 36.75% faster
wildmatch 3s 522156us
fnmatch   1s 544487us or 43.85% slower

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

t/t3070-wildmatch.sh

@@ -203,6 +203,10 @@ match 1 1 'XXX/adobe/courier/bold/o/normal//12/120/75/75/m/70/iso8859/1' 'XXX/*/
 match 0 0 'XXX/adobe/courier/bold/o/normal//12/120/75/75/X/70/iso8859/1' 'XXX/*/*/*/*/*/*/12/*/*/*/m/*/*/*'
 match 1 0 'abcd/abcdefg/abcdefghijk/abcdefghijklmnop.txt' '**/*a*b*g*n*t'
 match 0 0 'abcd/abcdefg/abcdefghijk/abcdefghijklmnop.txtz' '**/*a*b*g*n*t'
+match 0 x foo '*/*/*'
+match 0 x foo/bar '*/*/*'
+match 1 x foo/bba/arr '*/*/*'
+match 0 x foo/bb/aa/rr '*/*/*'
 
 pathmatch 1 foo foo
 pathmatch 0 foo fo
@@ -218,5 +222,9 @@ pathmatch 0 foo/bba/arr 'foo/*z'
 pathmatch 0 foo/bba/arr 'foo/**z'
 pathmatch 1 foo/bar 'foo?bar'
 pathmatch 1 foo/bar 'foo[/]bar'
+pathmatch 0 foo '*/*/*'
+pathmatch 0 foo/bar '*/*/*'
+pathmatch 1 foo/bba/arr '*/*/*'
+pathmatch 1 foo/bb/aa/rr '*/*/*'
 
 test_done

wildmatch.c

@@ -117,6 +117,18 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags)
 						return WM_NOMATCH;
 				}
 				return WM_MATCH;
+			} else if (!match_slash && *p == '/') {
+				/*
+				 * _one_ asterisk followed by a slash
+				 * with WM_PATHNAME matches the next
+				 * directory
+				 */
+				const char *slash = strchr((char*)text, '/');
+				if (!slash)
+					return WM_NOMATCH;
+				text = (const uchar*)slash;
+				/* the slash is consumed by the top-level for loop */
+				break;
 			}
 			while (1) {
 				if (t_ch == '\0')