userdiff: fix HTML hunk header regexp

Current HTML header regexp doesn't match headers without attributes.

So it fails to match <h1>...</h1>, while <h1 class="smth">...</h1> matches.

Make attributes optional to fix this.  The regexp is still far from
perfect, but now it at least handles the common case.

Signed-off-by: Ilya Kantor <iliakan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

userdiff.c

@@ -37,7 +37,7 @@ IPATTERN("fortran",
 	 "|//|\\*\\*|::|[/<>=]="),
 IPATTERN("fountain", "^((\\.[^.]|(int|ext|est|int\\.?/ext|i/e)[. ]).*)$",
 	 "[^ \t-]+"),
-PATTERNS("html", "^[ \t]*(<[Hh][1-6][ \t].*>.*)$",
+PATTERNS("html", "^[ \t]*(<[Hh][1-6]([ \t].*)?>.*)$",
 	 "[^<>= \t]+"),
 PATTERNS("java",
 	 "!^[ \t]*(catch|do|for|if|instanceof|new|return|switch|throw|while)\n"