This PR updates the git store to not register a change in a repository's
merge heads until conflicted paths are seen.

We currently use the repository's merge heads only to decide when the
list of conflicted paths should be refreshed. Previously, the logic
looked like this:

- Whenever we see a change in the merge heads, set the list of
conflicted paths by filtering the output of `git status`.

It turns out that when a conflicting merge takes a while, we can see
this sequence of events:

1. We get an event in .git and reload statuses and merge heads.
Previously there were no merge heads, and now we have some, but git
hasn't finished figuring out which paths have conflicts, so we set the
list of conflicted paths to `[]`.
2. Git finishes computing the list of conflicted paths, and we run
another scan that picks these up from `git status`, but then we throw
them away because the merge heads are the same as in (1).

By not updating our stored merge heads until we see some conflicts in
`git status`, we delay this step until (2), and so the conflicted paths
show up in the git panel as intended.

This means that our merge heads state no longer matches what's on disk
(in particular, during a clean merge we'll never update them at all),
but that's okay because we only keep this state for the purpose of
organizing conflicts.

Release Notes:

- Fixed a bug that could cause conflicted paths to not appear in their
own section in the git panel.