add alloc_ref_with_prefix()

add alloc_ref_with_prefix()

In three cases in remote.c, a "raw" ref is allocated using alloc_ref()
and then its is constructed using sprintf().  Clean it up by adding a
helper function, alloc_ref_with_prefix(), which creates a composite
name.  Use it in alloc_ref_from_str(), too, as it simplifies the code.

Open code alloc_ref() in alloc_ref_with_prefix(), as the former is
going to be removed in the patch after the next.

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