Ensure that all unmerged files are merged when conflict checking (#20528) (#20536)

Ensure that all unmerged files are merged when conflict checking (#20528) (#20536)

There is a subtle bug in the code relating to collating the results of
`git ls-files -u -z` in `unmergedFiles()`. The code here makes the
mistake of assuming that every unmerged file will always have a stage 1
conflict, and this results in conflicts that occur in stage 3 only being
dropped.

This PR simply adjusts this code to ensure that any empty unmergedFile
will always be passed down the channel.

The PR also adds a lot of Trace commands to attempt to help find future
bugs in this code.

Fix #19527

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: zeripath <art27@cantab.net>