Import fails for branches with regex-special-chars in names

[Saviq]

The branch names in import isn't escaped for regex-special-chars, an example with "++" in the branch name:

Traceback (most recent call last):
File "/usr/bin/git-bzr", line 709, in
sys.exit(main(sys.argv))
File "/usr/bin/git-bzr", line 703, in main
return func(argv[2:])
File "/usr/bin/git-bzr", line 576, in cmd_import
if branch_exists(branch):
File "/usr/bin/git-bzr", line 107, in branch_exists
matcher = re.compile(r'\s%s$' % branch)
File "/usr/lib/python2.7/re.py", line 190, in compile
return _compile(pattern, flags)
File "/usr/lib/python2.7/re.py", line 242, in _compile
raise error, v # invalid expression
sre_constants.error: multiple repeat