* Kokoro CI bots use git-sync-deps to get sources
Update git-sync-deps to reduce the amount of data downloaded on a first
checkout, while being able to checkout the specific commit specified in
the DEPS file.
Previously the CI bots would only clone --depth=1. But that's not
enough to check out a specific commit. So clone either blobless
or treeless. For a CI bot, treeless is preferable, because it
downloads the least data. For interactive use, blobless is better
because it prevents redundant downloads of tree data.
See
https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/Fixes: #5028
* --treeless decays to blob:none when git is too old
* Pin googletest to an older version, to make bazel build work
6b5a00eb2b