Bug 40009+40010: Add git_depth and git_branch options

Fixes #40009 (closed). Fixes #40010 (closed).

Tested with rbm fetch. Default behavior unchanged. Setting the optional options seems to have the desired effect. The tor project from tor-browser-build shrinks from 112 MiB to 47 MiB when using a single branch with depth 1. Haven't tried with a full build, but would be mildly surprised if it breaks things there. Happy to do other QA testing on request.