remote.c: fix "git push" weak match disambiguation

remote.c: fix "git push" weak match disambiguation

When "git push A:B" is given, and A (or B) is not a full refname
that begins with refs/, we require an unambiguous match with an
existing ref.  For this purpose, a match with a local branch or
a tag (i.e. refs/heads/A and refs/tags/A) is called a "strong
match", and any other match is called a "weak match".  A partial
refname is unambiguous when there is only one strong match with
any number of weak matches, or when there is only one weak match
and no other match.

However, as reported by Sparse with Ramsay Jones recently,
count_refspec_match() function had a bug where a variable in an
inner block masked a different variable of the same name, which
caused the weak matches to be ignored.

This fixes it, and adds tests for the fix.

Signed-off-by: Junio C Hamano <gitster@pobox.com>