bookmarks: name label for active bookmark correctly
Retain old label as well for backwards compatibility.
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.