builtin/fetch.c: ignore merge config when not fetching from branch's remote

builtin/fetch.c: ignore merge config when not fetching from branch's remote

When 'git fetch' is supplied a single argument, it tries to match it
against a configured remote and then fetch the refs specified by the
named remote's fetchspec.  Additionally, or alternatively, if the current
branch has a merge ref configured, and if the name of the remote supplied
to fetch matches the one in the branch's configuration, then git also adds
the merge ref to the list of refs to update.

If the argument to fetch does not specify a named remote, or if the name
supplied does not match the remote configured for the current branch, then
the current branch's merge configuration should not be considered.

git currently mishandles the case when the argument to fetch specifies a
GIT URL(i.e. not a named remote) and the current branch has a configured
merge ref.  In this case, fetch should ignore the branch's merge ref and
attempt to fetch from the remote repository's HEAD branch.  But, since
fetch only checks _whether_ the current branch has a merge ref configured,
and does _not_ check whether the branch's configured remote matches the
command line argument (until later), it will mistakenly enter the wrong
branch of an 'if' statement and will not fall back to fetch the HEAD branch.
The fetch ends up doing nothing and returns with a successful zero status.

Fix this by comparing the remote repository's name to the branch's remote
name, in addition to whether it has a configured merge ref, sooner, so that
fetch can correctly decide whether the branch's configuration is interesting
or not, and fall back to fetching from the remote's HEAD branch when
appropriate.

This fixes the test in t5510.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>