Retry failed repository clone/fetch

As seen on https://gitlab.com/gitlab-org/gitlab-ce/builds/6557396:

Fetching changes for fix/cycle-analytics-plan-issue with git depth set to 20...
[SNIP]
HEAD is now at ecbff8d Rephrase some system notes to be compatible with new system note style
error: RPC failed; HTTP 502 curl 22 The requested URL returned error: 502 Bad Gateway
fatal: The remote end hung up unexpectedly
ERROR: Build failed: exit code 1

And https://gitlab.com/gitlab-org/gitlab-ce/builds/6563389:

Cloning repository for master with git depth set to 20...
Cloning into '/builds/gitlab-org/gitlab-ce'...
error: RPC failed; HTTP 502 curl 22 The requested URL returned error: 502 Bad Gateway
fatal: The remote end hung up unexpectedly
ERROR: Build failed: exit code 1

/cc @ayufan @tmaczukin