creating test coverage for merge situation where there aren't actually conflicts, but there also aren't mdm repos on both sides.
extend those tests to cover when we use update in the middle of a merge.  don't think this is quite hitting all of the interesting behavior, though...
force actual merge commits to make sure we have merge heads.  this still doesn't trigger all of the fun behavior with checking multiple merge heads, though.
ah ha!  force update to have something to do during those merge commits.  now we're in the weeds: made the test fail with an NPE.
fix NPE when doing an update during a merge where one of the branches doesn't have the dependency.
changelog the mdm-update bugfix.

...it's such an odd series of steps to get there I have trouble putting it into a grammatically valid sentence.
Merge branch 'feature/merge-hash-check-fixes'