ExternalProject: Improve handling of git remote branches

ExternalProject handles git remote branches by commit hash. Due to
this, the git repository ends in detached states, and local commits
are discarded.

Use "git rebase" for remote branches instead of "git checkout".  If
there are uncommitted changes, use "git stash save/pop" to save changes
and restore them after the rebase.  If any of these operations fails,
try to restore the original status and exit with a fatal error, asking
the user to resolve the conflicts manually.

This also makes the behaviour of ExternalProject using git more similar
to the svn version, and probably more likely to what the user expects
by setting GIT_TAG to a branch.

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