Optimize prefixcmp()

Certain codepaths (notably "git log --pretty=format...") use
prefixcmp() extensively, with very short prefixes.  In those cases,
calling strlen() is a wasteful operation, so avoid it.

Initial patch by Marco Costalba.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -398,7 +398,11 @@ static inline int sane_case(int x, int high)
 
 static inline int prefixcmp(const char *str, const char *prefix)
 {
-	return strncmp(str, prefix, strlen(prefix));
+	for (; ; str++, prefix++)
+		if (!*prefix)
+			return 0;
+		else if (*str != *prefix)
+			return (unsigned char)*prefix - (unsigned char)*str;
 }
 
 static inline int strtoul_ui(char const *s, int base, unsigned int *result)