push: use same rules as git-rev-parse to resolve refspecs

push: use same rules as git-rev-parse to resolve refspecs

This commit changes the rules for resolving refspecs to match the
rules for resolving refs in rev-parse. git-rev-parse uses clear rules
to resolve a short ref to its full name, which are well documented.
The rules for resolving refspecs documented in git-send-pack were
less strict and harder to understand. This commit replaces them by
the rules of git-rev-parse.

The unified rules are easier to understand and better resolve ambiguous
cases. You can now push from a repository containing several branches
ending on the same short name.

Note, this may break existing setups. For example, "master" will no longer
resolve to "origin/master" even when there is no other "master" elsewhere.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>