libgit2 stores the lookup paths for gitconfig files in its global state.
This means that after a program changes its effective uid libgit2 will
still look for gitconfig files in the home directory of the original
uid. Expose a way to call C.git_libgit2_init so a user can reinitialize
the libgit2 global state.
(cherry picked from commit 3c5c580d78)