localrepo/branchcache: remove lbranchmap(), convert users to use utf-8 names
We don't need a "local-charset" aware branchmap() function, we can convert the
names when needed during the output.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'