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.

mercurial/context.py

@@ -336,17 +336,19 b' class basectx(object):'
 
         if listsubrepos:
             for subpath, sub in scmutil.itersubrepos(ctx1, ctx2):
+                try:
                 rev2 = ctx2.subrev(subpath)
-                try:
+                except KeyError:
+                    # A subrepo that existed in node1 was deleted between
+                    # node1 and node2 (inclusive). Thus, ctx2's substate
+                    # won't contain that subpath. The best we can do ignore it.
+                    rev2 = None
                     submatch = matchmod.narrowmatcher(subpath, match)
                     s = sub.status(rev2, match=submatch, ignored=listignored,
                                    clean=listclean, unknown=listunknown,
                                    listsubrepos=True)
                     for rfiles, sfiles in zip(r, s):
                         rfiles.extend("%s/%s" % (subpath, f) for f in sfiles)
-                except error.LookupError:
-                    self._repo.ui.status(_("skipping missing "
-                                           "subrepository: %s\n") % subpath)
 
         for l in r:
             l.sort()

tests/test-extdiff.t

@@ -126,6 +126,25 b' Check diff are made from the first paren'
   diff-like tools yield a non-zero exit code
 #endif
 
+issue3153: ensure using extdiff with removed subrepos doesn't crash:
+
+  $ hg init suba
+  $ cd suba
+  $ echo suba > suba
+  $ hg add
+  adding suba
+  $ hg ci -m "adding suba file"
+  $ cd ..
+  $ echo suba=suba > .hgsub
+  $ hg add
+  adding .hgsub
+  $ hg ci -Sm "adding subrepo"
+  $ echo > .hgsub
+  $ hg ci -m "removing subrepo"
+  $ hg falabala -r 4 -r 5 -S
+  diffing a.398e36faf9c6 a.5ab95fb166c4
+  [1]
+
 issue4463: usage of command line configuration without additional quoting
 
   $ cat <<EOF >> $HGRCPATH