filemerge: don't attempt to premerge change/delete conflicts This is mostly for completeness' sake -- at the moment we don't support any tools for change/delete conflicts that would do a premerge.

filemerge._mergecheck: add check for change/delete conflicts Merge tools that perform an actual 3-way merge can't handle change/delete conflicts. This adds a check for that.

filemerge._picktool: only pick from nomerge tools for change/delete conflicts For --tool or HGMERGE, we could have either: (a) proceeded with the particular tool, then failed the merge. (b) chosen to prompt regardless. We're explicitly choosing (b) here, because it's effectively what we've been doing so far and helps maintain an easier-to-use interface. However, in future patches we're going to change the default selection from 'pick changed version' to 'leave unresolved'. That fixes most of the brokenness involved with choice (b).

filemerge: add support for change/delete conflicts to the ':prompt' tool We haven't added the 'leave unresolved' option yet -- that will come in a future patch.

filemerge: add support for change/delete conflicts to the ':other' merge tool This, along with the previous patch to the :local merge tool, covers the full matrix of change/delete conflicts.

filemerge: add support for change/delete conflicts to the ':local' merge tool This covers two of the four cases of change/delete conflicts -- in an upcoming patch we'll make :other cover the other two.