Refactor the update logic to make it easier to follow. The following
fixes/improvements are some consequences of this change:
* Absorb a confusing git checkout failure message when the failure
is allowed and we act on that failure appropriately.
* Fix an unnecessary fetch in some scenarios when checking out a
git hash we already have locally.
* Stash and restore any local changes even when not rebasing.
* Avoid unsafe rebasing where we are not on a branch that is
already tracking the requested branch.
* When fetching, use --tags --force to ensure we get all the tags
and commits leading up to them regardless of whether the tags
are on branches or not. Also update our local tags if they move
on the remote.
Fixes: #20677
Commit 0aea435aa1 (ExternalProject: Provide choice of
git update strategies, 2020-02-12) added the git update
strategies, but the CHECKOUT strategy was not handling
remote refs correctly. The local ref would be checked out
instead and no warning or error would have been emitted.
The test that should have caught this was also malformed
and did not actually move the local master branch as intended.
The refactoring exposed that the original implementation
was referring to an undefined variable src_name, which was
previously only used in error messages. This has been fixed
as part of the refactoring work.
Fixes: #20336
