context: move reuse of context object to repo.__getitem__ (API)

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

--- a/mercurial/context.py	Thu Mar 29 22:22:51 2018 -0700
+++ b/mercurial/context.py	Thu Mar 29 22:51:45 2018 -0700
@@ -62,11 +62,6 @@
                 be committed,
     memctx: a context that represents changes in-memory and can also
             be committed."""
-    def __new__(cls, repo, changeid='', *args, **kwargs):
-        if isinstance(changeid, basectx):
-            return changeid
-
-        return super(basectx, cls).__new__(cls)
 
     def __bytes__(self):
         return short(self.node())
@@ -412,11 +407,6 @@
     def __init__(self, repo, changeid='.'):
         """changeid is a revision number, node, or tag"""
 
-        # since basectx.__new__ already took care of copying the object, we
-        # don't need to do anything in __init__, so we just exit here
-        if isinstance(changeid, basectx):
-            return
-
         if changeid == '':
             changeid = '.'
         self._repo = repo
@@ -2486,9 +2476,6 @@
     dateutil.parsedate() and defaults to current date, extra is a dictionary of
     metadata or is left empty.
     """
-    def __new__(cls, repo, originalctx, *args, **kwargs):
-        return super(metadataonlyctx, cls).__new__(cls, repo)
-
     def __init__(self, repo, originalctx, parents=None, text=None, user=None,
                  date=None, extra=None, editor=False):
         if text is None:

--- a/mercurial/localrepo.py	Thu Mar 29 22:22:51 2018 -0700
+++ b/mercurial/localrepo.py	Thu Mar 29 22:51:45 2018 -0700
@@ -782,6 +782,8 @@
     def __getitem__(self, changeid):
         if changeid is None:
             return context.workingctx(self)
+        if isinstance(changeid, context.basectx):
+            return changeid
         if isinstance(changeid, slice):
             # wdirrev isn't contiguous so the slice shouldn't include it
             return [context.changectx(self, i)