archive: look for first visible revision to build repo identity (issue4591)

No test for the case where all revisions are hidden because "archive" command
aborts if the target revision is null.

mercurial/archival.py

@@ -54,11 +54,17 b' def guesskind(dest):'
             return kind
     return None
 
+def _rootctx(repo):
+    # repo[0] may be hidden
+    for rev in repo:
+        return repo[rev]
+    return repo['null']
+
 def buildmetadata(ctx):
     '''build content of .hg_archival.txt'''
     repo = ctx.repo()
     base = 'repo: %s\nnode: %s\nbranch: %s\n' % (
-        repo[0].hex(), ctx.hex(), encoding.fromlocal(ctx.branch()))
+        _rootctx(repo).hex(), ctx.hex(), encoding.fromlocal(ctx.branch()))
 
     tags = ''.join('tag: %s\n' % t for t in ctx.tags()
                    if repo.tagtype(t) == 'global')

tests/test-obsolete.t

@@ -250,6 +250,30 b' We need to create a clone of 5 and add a'
   o  0:1f0dee641bb7 (public) [ ] add a
   
 
+  $ cd ..
+
+Revision 0 is hidden
+--------------------
+
+  $ hg init rev0hidden
+  $ cd rev0hidden
+
+  $ mkcommit kill0
+  $ hg up -q null
+  $ hg debugobsolete `getid kill0`
+  $ mkcommit a
+  $ mkcommit b
+
+Should pick the first visible revision as "repo" node
+
+  $ hg archive ../archive-null
+  $ cat ../archive-null/.hg_archival.txt
+  repo: 1f0dee641bb7258c56bd60e93edfa2405381c41e
+  node: 7c3bad9141dcb46ff89abf5f61856facd56e476c
+  branch: default
+  latesttag: null
+  latesttagdistance: 2
+  changessincelatesttag: 2
 
 
   $ cd ..