git svn: add an option to recode pathnames

git svn: add an option to recode pathnames

Introduce a new option 'svn.pathnameencoding' that instructs git svn to
recode pathnames to a given encoding.  It can be used by windows users
and by those who work in non-utf8 locales to avoid corrupted file names
with non-ascii characters.

[rp: renamed the option and added manpage documentation]

Signed-off-by: Dmitry Statyvka <dstatyvka@tmsoft-ltd.kiev.ua>
Signed-off-by: Robert Pollak <robert.pollak@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>