templatekw: add 'currentbookmark' keyword to show current bookmark easily

Before this patch, complicated template expression below is required
to show current active bookmark if it is associated with the
changeset.

    "{bookmarks % '{ifeq(bookmark, current, \"{bookmark}\")}'}"

This patch add 'currentbookmark' keyword to show current bookmark
easily.

mercurial/templatekw.py

@@ -208,6 +208,17 b' def showchildren(**args):'
     childrevs = ['%d:%s' % (cctx, cctx) for cctx in ctx.children()]
     return showlist('children', childrevs, element='child', **args)
 
+def showcurrentbookmark(**args):
+    """:currentbookmark: String. The active bookmark, if it is
+    associated with the changeset"""
+    import bookmarks as bookmarks # to avoid circular import issues
+    repo = args['repo']
+    if bookmarks.iscurrent(repo):
+        current = repo._bookmarkcurrent
+        if current in args['ctx'].bookmarks():
+            return current
+    return ''
+
 def showdate(repo, ctx, templ, **args):
     """:date: Date information. The date when the changeset was committed."""
     return ctx.date()
@@ -364,6 +375,7 b' keywords = {'
     'branches': showbranches,
     'bookmarks': showbookmarks,
     'children': showchildren,
+    'currentbookmark': showcurrentbookmark,
     'date': showdate,
     'desc': showdescription,
     'diffstat': showdiffstat,

tests/test-command-template.t

@@ -1850,6 +1850,15 b' Test current bookmark templating'
   2 bar* foo 
   1 
   0 
+  $ hg log --template "{rev} {currentbookmark}\n"
+  2 bar
+  1 
+  0 
+  $ hg bookmarks --inactive bar
+  $ hg log --template "{rev} {currentbookmark}\n"
+  2 
+  1 
+  0 
 
 Test stringify on sub expressions