archive: add baselen member to struct archiver_args

Calculate the length of base and save it in a new member of struct
archiver_args.  This way we don't have to compute it in each of the
format backends.

Note: parse_archive_args() guarantees that ->base won't ever be NULL.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive.h

@@ -6,6 +6,7 @@
 
 struct archiver_args {
 	const char *base;
+	size_t baselen;
 	struct tree *tree;
 	const unsigned char *commit_sha1;
 	const struct commit *commit;