completion: discover repo path in `__git_pseudoref_exists ()`

completion: discover repo path in `__git_pseudoref_exists ()`

The helper function `__git_pseudoref_exists ()` expects that the repo
path has already been discovered by its callers, which makes for a
rather fragile calling convention. Refactor the function to discover the
repo path itself to make it more self-contained, which also removes the
need to discover the path in some of its callers.

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