branch: strip whitespace before testing known branch name

Because dirstate._branch() strips leading/trailing spaces from .hg/branch,
"hg branch ' foo '" should abort if branch "foo" exists in another head.

tag command had a similar bug and fixed by 3d0a9c8d7184.

mercurial/commands.py

@@ -943,6 +943,9 b' def branch(ui, repo, label=None, **opts)'
 
     Returns 0 on success.
     """
+    if label:
+        label = label.strip()
+
     if not opts.get('clean') and not label:
         ui.write("%s\n" % repo.dirstate.branch())
         return

tests/test-branches.t

@@ -68,6 +68,18 b' invalid characters'
   abort: '\n' cannot be used in a name
   [255]
 
+trailing or leading spaces should be stripped before testing duplicates
+
+  $ hg branch 'b '
+  abort: a branch of the same name already exists
+  (use 'hg update' to switch to it)
+  [255]
+
+  $ hg branch ' b'
+  abort: a branch of the same name already exists
+  (use 'hg update' to switch to it)
+  [255]
+
 verify update will accept invalid legacy branch names
 
   $ hg init test-invalid-branch-name