sync: create dedicated manifest project update func

Cut out some more standalone code from Execute to make this func a
bit more manageable.  The manifest project update is pretty simple
and standalone, but still takes up a good chunk of what's left.

Change-Id: Idc2442d9def495eccd0a49cda203c44aef16f129
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/236614
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>