update: resurrect bare update from null parent to tip-most branch head

The situation is tricky if repository has no "default" branch, because "null"
revision belongs to non-existent "default" branch.

Before e1dd0de26557, bare update from null would bring us to the tip-most
non-closed branch head. e1dd0de26557 removed the special handling of missing
"default" branch since we wanted to stick to the uncommitted branch in that
case. But, if the parent is "null" revision, and if the missing branch is
"default", it shouldn't be an uncommitted branch. In this case, bare update
should bring us to the tip-most head as before.

This should fix the test breakage introduced by e1dd0de26557.

mercurial/destutil.py

@@ -101,6 +101,10 b' def _destupdatebranch(repo, clean, check'
             node = repo.revs('max(.::(%ln))', heads).first()
         if bookmarks.isactivewdirparent(repo):
             movemark = repo['.'].node()
+    elif currentbranch == 'default' and not wc.p1():
+        # "null" parent belongs to "default" branch, but it doesn't exist, so
+        # update to the tipmost non-closed branch head
+        node = repo.revs('max(head() and not closed())').first()
     else:
         node = repo['.'].node()
     return node, movemark, None

tests/test-newbranch.t

@@ -391,3 +391,75 b' We expect that bare update on new branch'
   commit: (new branch)
   update: (current)
   phases: 3 draft
+
+  $ cd ..
+
+We need special handling for repositories with no "default" branch because
+"null" revision belongs to non-existent "default" branch.
+
+  $ hg init nodefault
+  $ cd nodefault
+  $ hg branch -q foo
+  $ touch 0
+  $ hg ci -Aqm0
+  $ touch 1
+  $ hg ci -Aqm1
+  $ hg update -qr0
+  $ hg branch -q bar
+  $ touch 2
+  $ hg ci -Aqm2
+  $ hg update -qr0
+  $ hg branch -q baz
+  $ touch 3
+  $ hg ci -Aqm3
+  $ hg ci --close-branch -m 'close baz'
+  $ hg update -q null
+  $ hg log -GT'{rev} {branch}\n'
+  _  4 baz
+  |
+  o  3 baz
+  |
+  | o  2 bar
+  |/
+  | o  1 foo
+  |/
+  o  0 foo
+  
+
+ a) updating from "null" should bring us to the tip-most branch head as
+ there is no "default" branch:
+
+  $ hg update -q null
+  $ hg id -bn
+  -1 default
+  $ hg update
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  2 bar
+
+ b) but if we are at uncommitted "default" branch, we should stick to the
+ current revision:
+
+  $ hg update -q 0
+  $ hg branch default
+  marked working directory as branch default
+  $ hg id -bn
+  0 default
+  $ hg update
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  0 default
+
+ c) also, if we have uncommitted branch at "null", we should stick to it:
+
+  $ hg update -q null
+  $ hg branch new
+  marked working directory as branch new
+  $ hg id -bn
+  -1 new
+  $ hg update
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -bn
+  -1 new
+
+  $ cd ..