Branch extension bugs

Hi,

I have noticed a few bugs in branch extension:

1. Vcscommand cannot be enabled, unless fugitive or lawrencium is also
installed. That's because the code that loads branch extension
(extensions.vim, lines 167-170) doesn't check for vcscommand availability

2. Mercurial branch (as provided by lawrencium) is never displayed. That's
because b:mercurial_dir ends with a slash (unlike g:git_dir) and thus condition
in line 73 of branch.vim (in s:check_in_path()) is always false. This can be
fixed either by stripping the slash in line 58 or by removing :h in line 59 (I
believe the latter is better).