Add experimental support for passing a block to P4#run_resolve.
The
block is passed a P4::MergeData object encapsulating the context of
each merge performed. The block should evaluate to a string indicating
the desired result of the merge: 'ay', 'at', 'am', 's', etc. The
P4::MergeData object contains information about the files involved
in the merge and can invoke an external merge tool.
This is still experimental at this stage so the interface may
change as it evolves.