Add has_extension()

The little helper has_extension() documents through its name what we are
trying to do and makes sure we don't forget the underrun check.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-compat-util.h

@@ -139,6 +139,12 @@ static inline ssize_t xwrite(int fd, const void *buf, size_t len)
 	}
 }
 
+static inline int has_extension(const char *filename, int len, const char *ext)
+{
+	int extlen = strlen(ext);
+	return len > extlen && !memcmp(filename + len - extlen, ext, extlen);
+}
+
 /* Sane ctype - no locale, and works with signed chars */
 #undef isspace
 #undef isdigit