context: move reuse of context object to repo.__getitem__ (API)
As an example of how weird the basectx.__new__ is: whenever you create
a workingctx, basectx.__new__ gets called first. Since our __new__ has
a "changeid" argument as second parameter, when create the
workingctx(repo, text="blah"), the text gets bound to
"changeid". Since a string isn't a basectx, our __new__ ends up not
doing anything funny, but that's still very confusing code.
Another case is metadataonlyctx.__new__(), which I think exists in
order to prevent metadataonlyctx.__init__'s third argument
(originalctx) from being interpreted as a changeid in
basectx.__new__(), thereby getting reused.
Let's move this to repo.__getitem__ instead, where it will be pretty
obvious what the code does.
After this patch, changectx(ctx) will be an error (it will fail when
trying to see if it's a 20-byte string).
Differential Revision: https://phab.mercurial-scm.org/D2969
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