revset: do not fall through to revspec for literal: branch (issue4838)

If "literal:" is specified, it must not be a revset expression. It should
error out with a better message.

mercurial/revset.py

@@ -734,9 +734,12 b' def branch(repo, subset, x):'
         kind, pattern, matcher = util.stringmatcher(b)
         if kind == 'literal':
             # note: falls through to the revspec case if no branch with
-            # this name exists
+            # this name exists and pattern kind is not specified explicitly
             if pattern in repo.branchmap():
                 return subset.filter(lambda r: matcher(getbi(r)[0]))
+            if b.startswith('literal:'):
+                raise error.RepoLookupError(_("branch '%s' does not exist")
+                                            % pattern)
         else:
             return subset.filter(lambda r: matcher(getbi(r)[0]))
 

tests/test-revset.t

@@ -1328,6 +1328,9 b' we can use patterns when searching for t'
   $ log 'branch(unknown)'
   abort: unknown revision 'unknown'!
   [255]
+  $ log 'branch("literal:unknown")'
+  abort: branch 'unknown' does not exist!
+  [255]
   $ log 'branch("re:unknown")'
   $ log 'present(branch("unknown"))'
   $ log 'present(branch("re:unknown"))'