support/download/git: do not use bare clones

support/download/git: do not use bare clones

Currently, we are using bare clones, so as to minimise the disk usage,
most notably for largeish repositories such as the one for the Linux
kernel, which can go beyond the 1GiB barrier.

However, this precludes updating (and thus using) the submodules, if
any, of the repositories, as a working copy is required to use
submodules (becaue we need to know the list of submodules, where to find
them, where to clone them, what cset to checkout, and all those is
dependent upon the checked out cset of the father repository).

Switch to using /plain/ clones with a working copy.

This means that the extra refs used by some forges (like pull-requests
for Github, or changes for gerrit...) are no longer fetched as part of
the clone, because git does not offer to do a mirror clone when there is
a working copy.

Instead, we have to fetch those special refs by hand. Since there is no
easy solution to know whether the cset the user asked for is such a
special ref or not, we just try to always fetch the cset requested by
the user; if this fails, we assume that this is not a special ref (most
probably, it is a sha1) and we defer the check to the archive creation,
which would fail if the requested cset is missing anyway.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Matt Weber <matt@thewebers.ws>
Reviewed-by: Matt Weber <matt@thewebers.ws>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>