Patch application need not be on an unmodified file; applying to an
already changed file is supported provided the patch still applies
cleanly.  Add tests that modifies the contents of a file then applies
the patch and ensures that the patch applies cleanly, and the original
changes are also kept.

When building a recursive merge base, allow conflicts to occur.
Use the file (with conflict markers) as the common ancestor.

The user has already seen and dealt with this conflict by virtue
of having a criss-cross merge.  If they resolved this conflict
identically in both branches, then there will be no conflict in the
result.  This is the best case scenario.

If they did not resolve the conflict identically in the two branches,
then we will generate a new conflict.  If the user is simply using
standard conflict output then the results will be fairly sensible.
But if the user is using a mergetool or using diff3 output, then the
common ancestor will be a conflict file (itself with diff3 output,
haha!).  This is quite terrible, but it matches git's behavior.

Add a simple recursive test - where multiple ancestors exist and
creating a virtual merge base from them would prevent a conflict.