git fetcher: add support for rebaseable git repo

git fetcher: add support for rebaseable git repo

Some upstream git repo may rebase in the future, which means current
revision may disappear from the upstream repo after the rebase.

current git fetcher can not handle this case, because the git mirror
tar ball is per repo, and may also change in the rebase and lost the
current revision info.

To fix this issue, this patch
- add rebaseable tag in the SRC_URI
- for rebaseable repo, make git mirror tar ball per revision, in this
  case, even upstream rebase, the git mirror still has the current
  revision info.
- for rebaseable repo, generate mirror tar ball by default, since the
  repo may change in the future.

Signed-off-by: Yu Ke <ke.yu@intel.com>