tests: test that we can convert to manifestv2 with clone

This just adds a test to ensure that we can convert a manifestv1 repo to a
manifestv2 repo (for future commits only) by means of a local clone.

tests/test-manifestv2.t

@@ -1,4 +1,69 b''
-Check that entry is added to .hg/requires
+Create repo with old manifest
+
+  $ hg init existing
+  $ cd existing
+  $ echo footext > foo
+  $ hg add foo
+  $ hg commit -m initial
+
+We're using v1, so no manifestv2 entry is in requires yet.
+
+  $ grep manifestv2 .hg/requires
+  [1]
+
+Let's clone this with manifestv2 enabled to switch to the new format for
+future commits.
+
+  $ cd ..
+  $ hg clone --pull existing new --config experimental.manifestv2=1
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd new
+
+Check that entry was added to .hg/requires.
+
+  $ grep manifestv2 .hg/requires
+  manifestv2
+
+Make a new commit.
+
+  $ echo newfootext > foo
+  $ hg commit -m new
+
+Check that the manifest actually switched to v2.
+
+  $ hg debugdata -m 0
+  foo\x0021e958b1dca695a60ee2e9cf151753204ee0f9e9 (esc)
+
+  $ hg debugdata -m 1
+  \x00 (esc)
+  \x00foo\x00 (esc)
+  I\xab\x7f\xb8(\x83\xcas\x15\x9d\xc2\xd3\xd3:5\x08\xbad5_ (esc)
+
+Check that manifestv2 is used if the requirement is present, even if it's
+disabled in the config.
+
+  $ echo newerfootext > foo
+  $ hg --config experimental.manifestv2=False commit -m newer
+
+  $ hg debugdata -m 2
+  \x00 (esc)
+  \x00foo\x00 (esc)
+  \xa6\xb1\xfb\xef]\x91\xa1\x19`\xf3.#\x90S\xf8\x06 \xe2\x19\x00 (esc)
+
+Check that we can still read v1 manifests.
+
+  $ hg files -r 0
+  foo
+
+  $ cd ..
+
+Check that entry is added to .hg/requires on repo creation
 
   $ hg --config experimental.manifestv2=True init repo
   $ cd repo