util: replace slashes in branch names (#62)

git's fast-export doesn't seem to like slashes in branch names
("error: there are still refs under 'refs/hg/origin/branches/<name>").
Work around this by replacing slashes with _SLASH_.
Generalise the *_to_*_spaces() functions top name_*_to_*() functions,
using the mapping NAMES_HG_TO_GIT.
Add a test for this failure mode.