commit: correctly detect the start of the commit message

The end of the header is signaled by to consecutive LFs and the commit
message starts immediately after. Jumping over LFs at the start of the
message is a bug and leads to creating different commits if
when rebuilding history.

This also fixes an empty commit message being returned as "\n".
Merge pull request #1467 from carlosmn/commit-parse

commit: correctly detect the start of the commit message