error: add a structured exception for unsupported merge records We're going to use this in summary to print a better error message.

dirstate: back out 502b56a9e897 Superseded by the parent of this commit.

osutil: make statfiles check for interrupts periodically This is a simpler and faster fix for issue4878 than the contortions performed in 502b56a9e897.

osutil: don't leak on statfiles error Found using the power of the mighty eyeball.

histedit: extracts _isdirtywc function Checking if working copy is dirty was done in few places, this patch extracts it in _isdirtywc procedure.

node: add 'nullhex', hex-encoded nullid We're going to need this for upcoming changes, because the merge state stores nodes as hex strings.

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.