path: move `enter_repo()` into "setup.c"

path: move `enter_repo()` into "setup.c"

The function `enter_repo()` is used to enter a repository at a given
path. As such it sits way closer to setting up a repository than it does
with handling paths, but regardless of that it's located in "path.c"
instead of in "setup.c".

Move the function into "setup.c".

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