Always run Git.Config.store, so when the git config gets reloaded,
the override gets re-added to it, and changeGitRepo then calls extractGitConfig
on it and sees the annex.* settings from the override.
Remove any prior occurance of -c v and add it to the end. This way,
-c foo=1 -c foo=2 -c foo=1 will pass -c foo=1 to git, rather than -c foo=2
Note that, if git had some multiline config that got built up by
multiple -c's, this would not work still. But it never worked because
before the bug got fixed in the first place, the -c value was repeated
many times, so the multivalue thing would have been wrong. I don't think
-c can be used with multiline configs anyway, though git-config does
talk about them?
