narrow: add a TODO document

These are things that are bigger than we want to handle right now, but
are pretty important to get narrowing to be non-experimental.

Differential Revision: https://phab.mercurial-scm.org/D2196

hgext/narrow/TODO.rst

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+Integration with the share extension needs improvement. Right now
+we've seen some odd bugs, and the way we modify the contents of the
+.hg/shared file is unfortunate. See wrappostshare() and unsharenarrowspec().
+
+Resolve commentary on narrowrepo.wraprepo.narrowrepository.status
+about the filtering of status being done at an awkward layer. This
+came up the import to hgext, but nobody's got concrete improvement
+ideas as of then.
+
+Fold most (or preferably all) of narrowrevlog.py into core.
+
+Address commentary in narrowrevlog.excludedmanifestrevlog.add -
+specifically we should improve the collaboration with core so that
+add() never gets called on an excluded directory and we can improve
+the stand-in to raise a ProgrammingError.
+
+Figure out how to correctly produce narrowmanifestrevlog and
+narrowfilelog instances instead of monkeypatching regular revlogs at
+runtime to our subclass. Even better, merge the narrowing logic
+directly into core.
+
+Reason more completely about rename-filtering logic in
+narrowfilelog. There could be some surprises lurking there.
+
+Formally document the narrowspec format. Unify with sparse, if at all
+possible. For bonus points, unify with the server-specified narrowspec
+format.
+
+narrowrepo.setnarrowpats() or narrowspec.save() need to make sure
+they're holding the wlock.
+
+Implement a simple version of the expandnarrow wireproto command for
+core. Having configurable shorthands for narrowspecs has been useful
+at Google (and sparse has a similar feature from Facebook), so it
+probably makes sense to implement the feature in core. (Google's
+handler is entirely custom to Google, with a custom format related to
+bazel's build language, so it's not in the narrowhg distribution.)