template: add showbranch template for {branch}

Like showbranches, but always yields exactly one branch. Replaces the less
correct {branches|nonempty}.

mercurial/templatekw.py

@@ -145,6 +145,9 def getrenamedfn(repo, endrev=None):
 def showauthor(repo, ctx, templ, **args):
     return ctx.user()
 
+def showbranch(**args):
+    return args['ctx'].branch()
+
 def showbranches(**args):
     branch = args['ctx'].branch()
     if branch != 'default':
@@ -247,6 +250,7 def showtags(**args):
 # revcache - a cache dictionary for the current revision
 keywords = {
     'author': showauthor,
+    'branch': showbranch,
     'branches': showbranches,
     'children': showchildren,
     'date': showdate,

tests/test-command-template.t

@@ -570,7 +570,7 Issue2130: xml output for 'hg heads' is
 
 Keys work:
 
-  $ for key in author branches date desc file_adds file_dels file_mods \
+  $ for key in author branch branches date desc file_adds file_dels file_mods \
   >         file_copies file_copies_switch files \
   >         manifest node parents rev tags diffstat extras; do
   >     for mode in '' --verbose --debug; do
@@ -604,6 +604,33 Keys work:
   author--debug: other@place
   author--debug: A. N. Other <other@place>
   author--debug: User Name <user@hostname>
+  branch: default
+  branch: default
+  branch: default
+  branch: default
+  branch: foo
+  branch: default
+  branch: default
+  branch: default
+  branch: default
+  branch--verbose: default
+  branch--verbose: default
+  branch--verbose: default
+  branch--verbose: default
+  branch--verbose: foo
+  branch--verbose: default
+  branch--verbose: default
+  branch--verbose: default
+  branch--verbose: default
+  branch--debug: default
+  branch--debug: default
+  branch--debug: default
+  branch--debug: default
+  branch--debug: foo
+  branch--debug: default
+  branch--debug: default
+  branch--debug: default
+  branch--debug: default
   branches: 
   branches: 
   branches: