mergestate: add a constructor that sets up a clean merge state Eventually, we'll move the read call out of the constructor. This will: - avoid unnecessary reads when we're going to nuke the merge state anyway - avoid raising an exception if there's an unsupported merge record 'clean' seems like a good name for it because I wanted to avoid anything with the word 'new' in it, and 'reset' is more an action performed on a merge state than a way to get a new merge state. Thanks to Martin von Zweigbergk for feedback about naming this.

mergestate: raise structured exception for unsupported merge records We're going to catch this exception in 'hg summary' to print a better error message. This code is pretty untested, so there are no changes to test output. In upcoming patches we're going to test the output more thoroughly.

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dirstate: back out 502b56a9e897 Superseded by the parent of this commit.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

filectx: add isabsent method This will indicate whether this filectx represents a file that is *not* in a changectx. This will be used by merge and filemerge code to know about when a conflict is a change/delete conflict. While this is kind of hacky, it is the least bad of all the alternatives. Other options considered but rejected include: - isinstance(fctx, ...) -- not very Pythonic, doesn't support duck typing - fctx.size() is None -- the 'size()' call on workingfilectxes causes a disk stat - fctx.filenode() == nullid -- the semantics around filenode are incredibly confusing. In particular, for workingfilectxes, filenode() is always None no matter whether the file is present on disk or in either parent. Having different behavior for None versus nullid in the merge code is just asking for pain. Thanks to Pierre-Yves David for early review feedback here.