refactor fetch's ref matching to use refname_match()

refactor fetch's ref matching to use refname_match()

The old rules used by fetch were coded as a series of ifs.  The old
rules are:
1) match full refname if it starts with "refs/" or matches "HEAD"
2) verify that full refname starts with "refs/"
3) match abbreviated name in "refs/" if it starts with "heads/",
    "tags/", or "remotes/".
4) match abbreviated name in "refs/heads/"

This is replaced by the new rules
a) match full refname
b) match abbreviated name prefixed with "refs/"
c) match abbreviated name prefixed with "refs/heads/"

The details of the new rules are different from the old rules.  We no
longer verify that the full refname starts with "refs/".  The new rule
(a) matches any full string.  The old rules (1) and (2) were stricter.
Now, the caller is responsible for using sensible full refnames.  This
should be the case for the current code.  The new rule (b) is less
strict than old rule (3).  The new rule accepts abbreviated names that
start with a non-standard prefix below "refs/".

Despite this modifications the new rules should handle all cases as
expected.  Two tests are added to verify that fetch does not resolve
short tags or HEAD in remotes.

We may even think about loosening the rules a bit more and unify them
with the rev-parse rules.  This would be done by replacing
ref_ref_fetch_rules with ref_ref_parse_rules.  Note, the two new test
would break.

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