largefiles: download missing subrepo revs when archiving

This is something plain hgsubrepos already do.  Previously, an abort message
complained about an unknown revision.

hgext/largefiles/overrides.py

@@ -841,6 +841,7 b' def overridearchive(orig, repo, dest, no'
     archiver.done()
 
 def hgsubrepoarchive(orig, repo, ui, archiver, prefix, match=None):
+    repo._get(repo._state + ('hg',))
     rev = repo._state[1]
     ctx = repo._repo[rev]
 

tests/test-largefiles.t

@@ -1582,6 +1582,14 b' Lock in subrepo, otherwise the change is'
   lf_subrepo_archive/subrepo/large.txt
   lf_subrepo_archive/subrepo/normal.txt
 
+Test archiving a revision that references a subrepo that is not yet
+cloned (see test-subrepo-recursion.t):
+
+  $ hg clone -U . ../empty
+  $ cd ../empty
+  $ hg archive --subrepos -r tip ../archive.tar.gz 2>&1 | "$TESTDIR/filtercr.py"
+  cloning subrepo subrepo from $TESTTMP/statusmatch/subrepo
+  
   $ cd ..
 
 Test that addremove picks up largefiles prior to the initial commit (issue3541)