Update libgit2 submodule
Update libgit2 submodule

This is a non-released (i.e. non-tagged) version of libgit2 but it
contains a few important fixes on top of the most recent libgit2 0.21.x
release.

KDE/libqgit2,syntheticpp/libqgit2,syntheticpp/libqgit2,KDE/libqgit2,pav0n/libqgit2,pav0n/libqgit2