Rather than fetching the entire repository history, limit the fetch
depth to commits in the last 4 weeks. If a branch was branched longer
than 4 weeks ago, it seems reasonable to require it to be rebased before
it can be reviewed.

At the current rate of development, this reduces the bandwidth needed
for git pulls in the initial code checks from 73MB to 9MB.

Signed-off-by: Philip Withnall <withnall@endlessm.com>