BF: Avoid spurious backslashes in manpage double-dash option listings

[mih]

They were introduced to avoid rendering of typographic m-dashes instead
of double-dashes in the HTML version. This change limits this escaping
to the HTML version.

Fixes gh-4032

[yarikoptic]

shouldn't this be against maint?

$> git grep 'for p in parts)'
_datalad_build_support/formatters.py:            return ', '.join(p.replace('--', '-\\\\-') for p in parts)

$> git describe
0.12.4-9-gcc7e62771

[kyleam]

In my view a cosmetic fix like this is fine to be master only.

[mih]

In my view a cosmetic fix like this is fine to be master only.

I agree, but also wouldn't object if someone picks it up in maint. I admit that the choice of master wasn't a conscious one.