archive: add an experimental config to control the metadata file template

Experimental because given the possible complexity, it may be worth figuring out
how to load this from a file, similar to the style files for the log command,
instead of trying to stuff it on the command line.

mercurial/archival.py

@@ -97,7 +97,8 @@ def buildmetadata(ctx):
     )
 
     opts = {
-        'template': default
+        'template': repo.ui.config('experimental', 'archivemetatemplate',
+                                   default)
     }
 
     out = util.stringio()

tests/test-subrepo-deep-nested-change.t

@@ -263,6 +263,12 @@ A deleted subrepo file is flagged as dirty, like the top level repo
   $ hg update -Cq .
   $ rm -r ../wdir
 
+  $ hg archive -S -qr 'wdir()' ../wdir \
+  >            --config 'experimental.archivemetatemplate=archived {node|short}\n'
+  $ cat ../wdir/.hg_archival.txt
+  archived ffffffffffff
+  $ rm -r ../wdir
+
 .. but first take a detour through some deep removal testing
 
   $ hg remove -S -I 're:.*.txt' .