meson: inline the static 'git' library

When setting up `libgit.a` we first create the static library itself,
and then declare it as part of a dependency such that compile arguments,
include directories and transitive dependencies get propagated to the
users of that library. As such, the static library isn't expected to be
used by anything but the declared dependency.

Inline the static library so that we don't even use a separate variable
for it. This avoids any kind of confusion that may arise and clarifies
how the library is supposed to be used.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>