sync: Use 'git rebase' during 'repo sync --rebase'

'repo sync --rebase' should do a rebase if it encounters local commits
during a 'repo sync'.
This was broken by
https://gerrit-review.git.corp.google.com/c/git-repo/+/437421,
which caused this to execute the '_doff' hook (which stands for
'do fast forward'), which is implemented using 'git merge --no-stat'.

This caused *multiple* actual editor windows to pop up (*) during
'repo sync --rebase', asking the user to enter a commit message for the
merge.

In this CL I explicitly make that code path do a 'git rebase'.

(*) and if you use a terminal editor like 'vim', this means you have 2+ concurrent vim windows rendered in the same terminal, while 'repo sync' keeps on printing other output lines, again in the same terminal. The result is .... not pretty to say the least :(

Bug: b:434565811
Test: Used it myself for over a week.
Change-Id: I0bf3ff181f15b9d5b2e3f85f7f84e302139fdab7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/518602
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>

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