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.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda