merge: additional test cases to show merge-halting behavior
In the previous patches, we allowed the user to specify that
a merge process should be halted when a filemerge fails.
This patch adds tests that show additional places this logic
can be utilized -- via the options to do additional post-filemerge
checks to determine if a file merge was successful.
Differential Revision: https://phab.mercurial-scm.org/D952