Prepare for an interleaved fetch and checkout mode for `repo sync`. The
goal of the new mode is to significantly speed up syncs by running fetch
and checkout operations in parallel for different projects, rather than
waiting for all fetches to complete before starting any checkouts.
Bug: 421935613
Change-Id: I8c66d1e790c7bba6280e409b95238c5e4e61a9c8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/482821
Reviewed-by: Scott Lee <ddoman@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
85352825ff