prefix_filename: drop length parameter

This function takes the prefix as a ptr/len pair, but in
every caller the length is exactly strlen(ptr). Let's
simplify the interface and just take the string. This saves
callers specifying it (and in some cases handling a NULL
prefix).

In a handful of cases we had the length already without
calling strlen, so this is technically slower. But it's not
likely to matter (after all, if the prefix is non-empty
we'll allocate and copy it into a buffer anyway).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

abspath.c

@@ -246,9 +246,11 @@ char *absolute_pathdup(const char *path)
 	return strbuf_detach(&sb, NULL);
 }
 
-const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
+const char *prefix_filename(const char *pfx, const char *arg)
 {
 	static struct strbuf path = STRBUF_INIT;
+	size_t pfx_len = pfx ? strlen(pfx) : 0;
+
 #ifndef GIT_WINDOWS_NATIVE
 	if (!pfx_len || is_absolute_path(arg))
 		return arg;