properly handle detachted git worktrees

[pascalkuthe]

Right now we check if .git exists and is a directory to detect a git repo root.
This does not work well for git repos with multiple/detached work-tree where .git
is a file which contains the location of the folder that contains files that would normally be located in .git.

This PR changes all checks for .git to use .exists() instead of .is_dir(), so it also works for files.
Since the git cli uses the same detection mechanism (try tough .git && git status in an empty directory)
this seems more inline with the intended behavior.

Furthermore, workflows with multiple worktrees become much nicer with this (I use one for each branch to quickly context switch)
as the file picker now stops at the correct location.
Fixing this will also be important for getting repo caching to work correctly in the future.

This change is technically a breaking for repos that contain random .git files but as this messes with
the git cli as well, so I think those cases can be ignored and I did not mark it as a breaking change.

[pascalkuthe]

This should also indirectly fix #983 I believe. This does not remove the limit as requested there or make the picker streaming/async but this should cover the specific usecase of detecting git worktrees as described in that issue.

[wezm]

Thanks, this does fix #983 for me.