synchronize source repos concurrently

[edmundnoble]

Synchronizes source repos concurrently with 4 repos being synchronized at once. I couldn't find a good way to make this number a flag, and so I figured that if someone wants to do that, they can do it later, because doing this concurrently is already a significant speedup for me and seems to be mostly free.

I didn't write any tests, because I figured this is guaranteed to be correct and I don't know how I'd even test it.

This PR depends on #9843 just because that PR makes it more obvious that sequenceConcurrentlyBoundedRebuild is correct.

QA Notes

Ran this on a work project which has 12 git source repositories. Observed the interleaved output from git and the much faster build from clean.

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)