remote: use the configured default branch name when appropriate

remote: use the configured default branch name when appropriate

When guessing the default branch name of a remote, and there are no refs
to guess from, we want to go with the preference specified by the user
for the fall-back, i.e. the default name to be used for the initial
branch of new repositories (because as far as the user is concerned, a
remote that has no branches yet is a new repository).

At the same time, when talking to an older Git server that does not
report a symref for `HEAD` (but instead reports a commit hash), let's
try to guess the configured default branch name first. If it does not
match the reported commit hash, let's fall back to `master` as before.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>