filemerge: add a 'leave unresolved' option to regular prompts 'Regular' here means anything that isn't a change/delete prompt. We'll add this option to change/delete prompts in a subsequent patch.

filemerge: add debug output for whether this is a change/delete conflict Just like binary and symlink conflicts, change/delete conflicts influence the tool picked.

webcommands: test that fctx is not None in filediff() A block of code above this one already says "if fctx is not None", and it's also what this code actually intends to check, so let's be specific as PEP-8 recommends.