ExternalProject: Improve robustness of update step

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

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