bookmarks: warn about bookmark names that unambiguously resolve to a node (BC)

I just burned myself on this today because I left out the -r in my `hg
bookmark` command, which then left me confused because I didn't notice
the bookmark I created in the wrong place that was silently shadowing
the revision I was trying to check out. Let's warn the user.

This patch only enforces the check on bookmark names 4 characters long
or longer. We can tweak that if we'd like, I selected that since
that's the fewest characters shortest will use in the templater
output.

A previous version of this patch rejected such bookmarks. It was
proposed during review (and I agree) that the behavior change for a
bookmark named "cafe" or similar as history accumulated was a little
too weird, but that the warning definitely has merit.

mercurial/commands.py

@@ -968,6 +968,11 def bookmark(ui, repo, *names, **opts):
             and not force):
             raise error.Abort(
                 _("a bookmark cannot have the name of an existing branch"))
+        if len(mark) > 3 and mark in repo and not force:
+            repo.ui.warn(
+                _("bookmark %s matches a changeset hash\n"
+                  "(did you leave a -r out of an 'hg bookmark' command?)\n") %
+                mark)
 
     if delete and rename:
         raise error.Abort(_("--delete and --rename are incompatible"))

tests/test-bookmarks.t

@@ -311,6 +311,15 bookmark with integer name
   abort: cannot use an integer as a name
   [255]
 
+bookmark with a name that matches a node id
+  $ hg bookmark 925d80f479bb db815d6d32e6
+  bookmark 925d80f479bb matches a changeset hash
+  (did you leave a -r out of an 'hg bookmark' command?)
+  bookmark db815d6d32e6 matches a changeset hash
+  (did you leave a -r out of an 'hg bookmark' command?)
+  $ hg bookmark -d 925d80f479bb
+  $ hg bookmark -d db815d6d32e6
+
 incompatible options
 
   $ hg bookmark -m Y -d Z