doc/README

extdiff: correctly handle deleted subrepositories (issue3153)

Previously, when extdiff was called on two changesets where
a subrepository had been removed, an unexpected KeyError would
be raised.

Now, the missing subrepository will be ignored.  This behavior
mirrors the behavior in diffordiffstat from cmdutil.py line
~1138-1153.  The KeyError is caught and the revision is
set to None.

try/catch of LookupError around matchmod.narrowmatcher and
sub.status is removed, as LookupError is not raised anywhere
within those methods or deeper calls.

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