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>

Documentation/git-send-pack.txt

@@ -85,7 +85,9 @@ Each pattern pair consists of the source side (before the colon)
 and the destination side (after the colon).  The ref to be
 pushed is determined by finding a match that matches the source
 side, and where it is pushed is determined by using the
-destination side.
+destination side. The rules used to match a ref are the same
+rules used by gitlink:git-rev-parse[1] to resolve a symbolic ref
+name.
 
  - It is an error if <src> does not match exactly one of the
    local refs.