Previously, we only enabled merge conflict parsing for files that were
unmerged at the last time a change was detected to the repo's merge
heads. Now we enable the parsing for these files *and* any files that
are currently unmerged.

The old strategy meant that conflicts produced via `git stash pop` would
not be parsed.

Release Notes:

- Fixed parsing of merge conflicts when the conflict was produced by a
`git stash pop`