toaster: typeaheads Add a git revisions suggestions

toaster: typeaheads Add a git revisions suggestions

When we're importing a layer it's useful to suggest available git revisions of the
layers. This uses git ls-remote to fetch the revisions and then filter on this.
Caching is added for this typeahead to avoid having to fetch this
information multiple times in a single session.

[YOCTO #8429]

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>