bookmarks: allow (re-)activating a bookmark on the current changeset

Allow a bookmark that points to the current changeset to be made the
active bookmark without requiring --force. Previously, this would've
aborted with:

  abort: bookmark 'Z' already exists (use -f to force)

mercurial/commands.py

@@ -811,6 +811,9 b' def bookmark(ui, repo, mark=None, rev=No'
     def checkconflict(repo, mark, force=False, target=None):
         if mark in marks and not force:
             if target:
+                if marks[mark] == target and target == cur:
+                    # re-activating a bookmark
+                    return
                 anc = repo.changelog.ancestors([repo[target].rev()])
                 bmctx = repo[marks[mark]]
                 if bmctx.rev() in anc:

tests/test-bookmarks.t

@@ -363,6 +363,11 b' test rollback'
    * Z                         2:db815d6d32e6
      x  y                      2:db815d6d32e6
 
+activate bookmark on working dir parent without --force
+
+  $ hg bookmark --inactive Z
+  $ hg bookmark Z
+
 test clone
 
   $ hg bookmark -r 2 -i @