Support shallow clones

[bananer]

Passing --depth 1 to repo init causes all modules to be shallow clones. A quick test showed that this reduces the required disk space and time to download significantly - less than 100GB for the whole 18.1 tree before build.

I want to support this, but only as an opt-in, since it might break some peoples' workflow which assumes the whole history to be present.

[Robin-Sch]

Is it possible to "update" shallow clones by only downloading the new changes, without re-downloading everything?

[petefoth]

Fixed by #431

[petefoth]

#431 did not fix this, and managed to break the build instead - see #478

Re-opened - the fix from #431 needs to be corrected (change ... repo init "$REPO_INIT_ARGS" ... to ... repo init "${REPO_INIT_ARGS}" ...

This issue can stay open till we get a PR that works ;)