git/daemon: Update "branch" property when we receive index change signal

Fixes #1490 (closed)

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The "branch changed" was already detected and the appropriate "changed" was already triggered by the daemon. It just didn't reload the branch name (and probably other things which aren't covered by this MR).

I am pretty sure this is not the correct way of fixing it, as it circumvents a bit the "changed" signal :/ Also, I don't like the fact that since the build directory contains the branch name, then making a new feature branch will require triggering a full rebuild. I mean, it might make sense to have separate build directories per architecture, but less for branches because anyway meson manages to rebuild properly without messing up with old object files or whatever. So I'm not much convinced about all that here…