rebase -r: let `label` generate safer labels

rebase -r: let `label` generate safer labels

The `label` todo command in interactive rebases creates temporary refs
in the `refs/rewritten/` namespace. These refs are stored as loose refs,
i.e. as files in `.git/refs/rewritten/`, therefore they have to conform
with file name limitations on the current filesystem in addition to the
accepted ref format.

This poses a problem in particular on NTFS/FAT, where e.g. the colon,
double-quote and pipe characters are disallowed as part of a file name.

Let's safeguard against this by replacing not only white-space
characters by dashes, but all non-alpha-numeric ones.

However, we exempt non-ASCII UTF-8 characters from that, as it should be
quite possible to reflect branch names such as `↯↯↯` in refs/file names.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>