meson: install the git hook (#6425)

meson: install the git hook (#6425)

This was done autogen.sh previously and was dropped in
72cdb3e783174dcf9223a49f03e3b0e2ca95ddb8. Let's add it back.
The meson configuration step is the only reasonable place.

Note that this only works for the most standard git dirs, e.g.
the hook will not be installed if git worktree is used or if
$GIT_DIR is specified, etc. I think that's OK because most of
the time meson will be run at least once in the original cloned
dir.