convert bare readlink to strbuf_readlink
convert bare readlink to strbuf_readlink

This particular readlink call never NUL-terminated its
result, making it a potential source of bugs (though there
is no bug now, as it currently always respects the length
field). Let's just switch it to strbuf_readlink which is
shorter and less error-prone.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git