[update-checkout] Add --match-timestamp mode to sync adjacent repos while bisecting.

[graydon]

This adds a new command line flag to update-checkout to enable syncing adjacent
repositories to the swift repo by commit timestamp, as one needs to do when
bisecting swift history.

To use it, one just runs utils/update-checkout --match-timestamp; it extracts the
current checkout's timestamp and then moves the adjacent repositories to the latest
checkout they have before that timestamp.

If there is a most-recent commit before the target timestamp made by swift-ci, it will
prefer that over a more-recent commit not-by-swift-ci; this is necessary to land on a
revision that actually represents a valid merged state, rather than a more-recent
merge input on an upstream branch (as happens every few minutes in the llvm and clang
repos).

[jrose-apple]

Rather than look at the most-recent commit by swift-ci, you should be able to look for the most recent mainline commit. I don't remember the git magic for that offhand, though.

[graydon]

(Conferred with Jordan on this, he means --first-parent, which is unfortunately not stable enough to rely on in our history, so merging as-is)

[graydon]

@swift-ci please smoke test and merge

[gottesmm]

This looks pretty cool

[graydon]

@swift-ci please smoke test and merge