tag: clarify warning about making a tag on a branch head

Currently the warning is ambiguous about whether the new tag (possibly specified
via --rev) is being added on a branch head or whether the working directory is
based on a branch head. Clarify the error message to eliminate this ambiguity.

mercurial/commands.py

@@ -7077,7 +7077,8 def tag(ui, repo, name1, *names, **opts)
                 raise error.Abort(_('uncommitted merge'))
             bheads = repo.branchheads()
             if not opts.get('force') and bheads and p1 not in bheads:
-                raise error.Abort(_('not at a branch head (use -f to force)'))
+                raise error.Abort(_('working directory is not at a branch head '
+                                    '(use -f to force)'))
         r = scmutil.revsingle(repo, rev_).node()
 
         if not message:

tests/test-tag.t

@@ -109,7 +109,7 tagging on a non-head revision
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg tag -l localblah
   $ hg tag "foobar"
-  abort: not at a branch head (use -f to force)
+  abort: working directory is not at a branch head (use -f to force)
   [255]
   $ hg tag -f "foobar"
   $ cat .hgtags
@@ -339,7 +339,7 tagging on null rev
   $ hg up null
   0 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ hg tag nullrev
-  abort: not at a branch head (use -f to force)
+  abort: working directory is not at a branch head (use -f to force)
   [255]
 
   $ hg init empty