git-svn: fix signed commit parsing

git-svn: fix signed commit parsing

When parsing a commit object, git-svn wrongly think that a line
containing spaces means the end of headers and the start of the commit
message. In case of signed commit, the gpgsig entry contains a line with
one space, so "git svn dcommit" will include part of the signature in
the commit message.

An example of such problem :
http://svnweb.mageia.org/treasurer?view=revision&revision=86

This commit changes the regex to only match an empty line as separator
between the headers and the commit message.

Signed-off-by: Nicolas Vigier <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Eric Wong <redacted>