remote: make refspec follow the same disambiguation rule as local refs

remote: make refspec follow the same disambiguation rule as local refs

When matching a non-wildcard LHS of a refspec against a list of
refs, find_ref_by_name_abbrev() returns the first ref that matches
using any DWIM rules used by refname_match() in refs.c, even if a
better match occurs later in the list of refs.

This causes unexpected behavior when (for example) fetching using
the refspec "refs/heads/s:<something>" from a remote with both
"refs/heads/refs/heads/s" and "refs/heads/s"; even if the former was
inadvertently created, one would still expect the latter to be
fetched.  Similarly, when both a tag T and a branch T exist,
fetching T should favor the tag, just like how local refname
disambiguation rule works.  But because the code walks over
ls-remote output from the remote, which happens to be sorted in
alphabetical order and has refs/heads/T before refs/tags/T, a
request to fetch T is (mis)interpreted as fetching refs/heads/T.

Update refname_match(), all of whose current callers care only if it
returns non-zero (i.e. matches) to see if an abbreviated name can
mean the full name being tested, so that it returns a positive
integer whose magnitude can be used to tell the precedence, and fix
the find_ref_by_name_abbrev() function not to stop at the first
match but find the match with the highest precedence.

This is based on an earlier work, which special cased only the exact
matches, by Jonathan Tan.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>