tests: add test for widening from an empty clone

Narrow clones that track no paths currently don't even include the
root manifest (which is the only manifest when using flat
manifests). That means that when we widen from such a clone, we need
to make sure that we send the root manifest (and other manifests if
using tree manifests). That currently works because we always resend
all manifest that match the new narrowspec. However, we're about to
stop resending manifests that the client already has and there's a
risk of this breaking then, so let's add a test.

Differential Revision: https://phab.mercurial-scm.org/D5143

tests/test-narrow-widen-no-ellipsis.t

@@ -30,17 +30,30 b''
 
 narrow clone the inside file
 
-  $ hg clone --narrow ssh://user@dummy/master narrow --include inside
+  $ hg clone --narrow ssh://user@dummy/master narrow
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
-  added 3 changesets with 1 changes to 1 files
+  added 3 changesets with 0 changes to 0 files
   new changesets *:* (glob)
   updating to branch default
-  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ cd narrow
   $ hg tracked
+  $ hg files
+  [1]
+
+widen from an empty clone
+
+  $ hg tracked --addinclude inside
+  comparing with ssh://user@dummy/master
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 0 changesets with 1 changes to 1 files
+  $ hg tracked
   I path:inside
   $ ls
   inside