svn: properly escape arguments for authors-prog

svn: properly escape arguments for authors-prog

Previously, the call to authors-prog was not properly escaped, so any
special characters in the Subversion username, such as spaces and
semi-colons, would be interpreted by the shell rather than being passed
in as the first argument.  Now all unsafe characters are escaped using
"git rev-parse --sq-quote"

[ew: switched from "\Q..\E" to "rev-parse --sq-quote"]

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>