repo_pull.py: Optimize pulls for shallow-cloned projects

The script would previously unshallow repositories
with a clone-depth set in the manifest, causing slow pulls.

This patch reads the clone-depth attribute and uses
`git fetch --depth` and `git checkout` for those specific projects,
avoiding the performance issue.

Bug: 429538153
Test: development/tools/repo_pull/repo_pull.py bash \
      '{topic} is:open' -g {gerrit_host}
Change-Id: I10adf644cd5c0379d611145227b4a0046461a15c
1 file changed