templatekw: introduce activebookmark keyword

Today, the terms 'active' and 'current' are interchangeably used throughout the
codebase in reference to the active bookmark (the bookmark that will be updated
with the next commit). This leads to confusion among developers and users.
This patch is part of a series to standardize the usage to 'active' throughout
the mercurial codebase and user interface.

mercurial/templatekw.py

@@ -221,8 +221,14 b' def showchildren(**args):'
     childrevs = ['%d:%s' % (cctx, cctx) for cctx in ctx.children()]
     return showlist('children', childrevs, element='child', **args)
 
+# Deprecated, but kept alive for help generation a purpose.
 def showcurrentbookmark(**args):
     """:currentbookmark: String. The active bookmark, if it is
+    associated with the changeset (DEPRECATED)"""
+    return showactivebookmark(**args)
+
+def showactivebookmark(**args):
+    """:activetbookmark: String. The active bookmark, if it is
     associated with the changeset"""
     import bookmarks as bookmarks # to avoid circular import issues
     repo = args['repo']
@@ -418,12 +424,14 b' def showtags(**args):'
 # cache - a cache dictionary for the whole templater run
 # revcache - a cache dictionary for the current revision
 keywords = {
+    'activebookmark': showactivebookmark,
     'author': showauthor,
     'bisect': showbisect,
     'branch': showbranch,
     'branches': showbranches,
     'bookmarks': showbookmarks,
     'children': showchildren,
+    # currentbookmark is deprecated
     'currentbookmark': showcurrentbookmark,
     'date': showdate,
     'desc': showdescription,