added recursive repository support for golang

[paulhfischer]

resolves #1788

note that i also added depth=1 to reduce cache size and installation time. not sure if the whole command call should be replaced with _shallow_clone, depends on whether you want to keep all command calls within the helper module or not.

[asottile]

oh :/ this was going to be my stream today -- this needs a test

[paulhfischer]

oh i'm sorry :/

do you want to do this later today or shall i add a test?

[asottile]

up to you -- the test is the actually interesting part of the patch. you might as well proceed

--depth actually doesn't matter here since that directory is ephemeral, though --reference might make it faster

[asottile]

I did some tests with --shared and --reference and --depth=1 and they didn't make a difference in the cloned size so I'm just going to stick to the easiest command here