Signed-off-by: Petr Štetiar <ynezz@true.cz>

Switching branches and applying patches in the build repos will
unnecessarily touch many files, causing rebuilds of packages that didn't
really change; furthermore, it is filling the reflog with many entries.

Don't ever switch to base branch in the build repos and apply patches in
a temporary clone to avoid these issues.

In addition, GPG signing is generally disabled in the build repos to
override potential global configuration (as signing doesn't make sense and
will slow down rebases).

Suggested-by: Steffen Möller <steffen_moeller@gmx.de>

This option will make the generated commit IDs deterministic, greatly
reducing the number of repository objects created when calling `make
update` repeatedly.

Using a loop makes the script a bit nicer, and it works around a git bug
corrupting patch dates when multiple patches are applied at once.

This fixes `make update` not working correctly when GLUON_SITEDIR is set
manually.