subrepo: pull revisions on demand when archiving hg subrepos

Before, running 'hg archive -S' could result in

 abort: unknown revision '65903cebad86f1a84bd4f1134f62fa7dcb7a1c98'!

if a subrepo was missing completely or had missing changesets. Now,
the missing changesets will be pulled or cloned as appropriate.

This make Mercurial subrepos match Git subrepos which already took
care to fetch any missing commits before starting the archive.

mercurial/subrepo.py

@@ -418,6 +418,7 b' class hgsubrepo(abstractsubrepo):'
                                % (inst, subrelpath(self)))
 
     def archive(self, ui, archiver, prefix):
+        self._get(self._state + ('hg',))
         abstractsubrepo.archive(self, ui, archiver, prefix)
 
         rev = self._state[1]

tests/test-subrepo-recursion.t

@@ -304,6 +304,47 b' Test archiving to zip file (unzip output'
   archiving (foo/bar) [================================>] 1/1
                                                               \r (esc)
 
+Test archiving a revision that references a subrepo that is not yet
+cloned:
+
+  $ hg clone -U . ../empty
+  $ cd ../empty
+  $ hg archive --subrepos -r tip ../archive.tar.gz 2>&1 | $TESTDIR/filtercr.py
+  
+  archiving [                                           ] 0/3
+  archiving [                                           ] 0/3
+  archiving [=============>                             ] 1/3
+  archiving [=============>                             ] 1/3
+  archiving [===========================>               ] 2/3
+  archiving [===========================>               ] 2/3
+  archiving [==========================================>] 3/3
+  archiving [==========================================>] 3/3
+                                                              
+  archiving (foo) [                                     ] 0/3
+  archiving (foo) [                                     ] 0/3
+  archiving (foo) [===========>                         ] 1/3
+  archiving (foo) [===========>                         ] 1/3
+  archiving (foo) [=======================>             ] 2/3
+  archiving (foo) [=======================>             ] 2/3
+  archiving (foo) [====================================>] 3/3
+  archiving (foo) [====================================>] 3/3
+                                                              
+  archiving (foo/bar) [                                 ] 0/1
+  archiving (foo/bar) [                                 ] 0/1
+  archiving (foo/bar) [================================>] 1/1
+  archiving (foo/bar) [================================>] 1/1
+                                                              
+  cloning subrepo foo from $TESTTMP/repo/foo
+  cloning subrepo foo/bar from $TESTTMP/repo/foo/bar
+  
+The newly cloned subrepos contain no working copy:
+
+  $ hg -R foo summary
+  parent: -1:000000000000  (no revision checked out)
+  branch: default
+  commit: (clean)
+  update: 4 new changesets (update)
+
 Disable progress extension and cleanup:
 
   $ mv $HGRCPATH.no-progress $HGRCPATH