sync: superproject performance changes.

After updating all project’s revsionIds with the SHAs from superproject,
write the updated manifest into superproject_override.xml file. Reload
that file for future Reloads. This file is created in exp-superproject
directory.

Moved most of the code that is superproject specific into
git_superproject.py and wrote test code.

If git pull fails, did a git clone of the superproject.

We saw performance gains for consecutive repo sync's. The time to sync
went down from around 120 secs to 40 secs when repo sync is executed
consecutively.

Tested the code with the following commands.

$ ./run_tests -v tests/test_git_superproject.py
$ ./run_tests -v

Tested the sync code by copying all the repo changes into my Android
AOSP checkout and doing a repo sync --use-superproject twice.

First run
$ time repo sync --use-superproject
...
real	21m3.745s
user	97m59.380s
sys	19m11.286s

After two consecutive sync runs
$ time repo sync -c -j8 --use-superproject
real	0m39.626s
user	0m29.937s
sys	0m38.155s

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707


Tested-by: Raman Tenneti <rtenneti@google.com>

Change-Id: Id79a0d7c4d20babd65e9bd485196c6f8fbe9de5e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296082


Reviewed-by: Ian Kasprzak <iankaz@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>