git-svn: Teach dcommit --mergeinfo to handle multiple lines

git-svn: Teach dcommit --mergeinfo to handle multiple lines

"svn dcommit --mergeinfo" replaces the svn:mergeinfo property in an
upstream SVN repository with the given text. The svn:mergeinfo
property may contain commits originating on multiple branches,
separated by newlines.

Cause space characters in the mergeinfo to be replaced by newlines,
allowing a user to create history representing multiple branches being
merged into one.

Update the corresponding documentation and add a test for the new
functionality.

Signed-off-by: Bryan Jacobs <bjacobs@woti.com>
Acked-by: Sam Vilain <sam@vilain.net>
Acked-by: Eric Wong <normalperson@yhbt.net>