UpdateRemoteMirror: handle large number of branches

Fixes #1878 (closed)

This bug is about listing so many branches on the command line of a git push command that the operating system complains about the command line being too long.

I managed to reproduce the problem by getting the OS to complain, but this required such a large payload that the resulting test took 30 seconds to run. That seems out of proportion. So instead of a "proper" bug-reproducing test, I ended up writing unit tests that check that we chunk the number of branches pushed.

The original bug was only about deleting remote branches but the same problem exists, potentially, for creating/updating branches. So I updated that part of the code too.