git-gui: do not change global vars in choose_repository::pick

git-gui: do not change global vars in choose_repository::pick

The repository picker (choose_repository::pick, AKA pick) on success
always returns with the current directory at the root of the selected
worktree, with the global variable _gitdir holding the name of the
git repository, possibly as a relative path, and _prefix {}. The
worktree root (_gitworktree) is not filled out, and if the selection was
from the "recent" list, no validation has occurred beyond testing that
the worktree root exists. So, repository and worktree validation are
still needed to be sure the new repo + worktree is usable.

pick only supports worktrees with a .git entry in the worktree root, so
git repository and worktree discovery will work starting in the current
directory on return. In cases of error, or user abort, pick exits the
process rather than returning.

So, let's change pick to not alter any global values, with success
indicated by the process returning to the caller. In this case, the
current directory is the worktree root, with a .git entry. The caller
then proceeds with normal discovery to find and validate both repository
and worktree.

With this, pick now returns 1 in the success case, but additional work
would be necessary to return from conditions where 0 should be returned.
Checking this return value would be superfluous.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>