filemerge: introduce class whose objects represent files not in a context
Most code is going to barf at the return values here (particularly from data
and size), so we restrict it to the filemerge code.
This is already somewhat supported via:
ctx.filectx(f, fileid=nullid)
Indeed, for add/add conflicts (ancestor doesn't have the file) we use precisely
that. However, that is broken in subtle ways:
- The cmp() function in filectx returns False (identical) for such a filectx
when compared to a zero-length file.
- size() returns 0 rather than some sort of value indicating that the file isn't
present.
- data() returns '' rather than some sort of value indicating that the file isn't
present.
Given the relatively niche use of such filectxes, this seems to be the simplest
way to fix all these issues.
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