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 <boklm@mars-attacks.org>
rev-parse doc: clarify use of optional / required arguments

"the option is optional" was confusing as it is not the option but its
argument which is optional.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
config doc: user.signingkey is also used for signed commits

The description of the user.signingkey option only mentioned its use
when creating a signed tag. Make it clear that is is also used when
creating signed commits.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>