gitlog2gnucl had several issues.  It was emitting more context than
summary line as the preamble of the ChangeLog entry.  On commit logs
with no clear summary line (i.e, pre-git commit logs) the ChangeLog
entry could be empty, rather than having the first line of the commit
log.  Signed-off and commit hashes were included in the ChangeLog
entry.

This patch fixes those issues.

	* gitlog2gnucl (Entry::first_msg_line): New data member.
	(Entry::initialize): Adjust to initialize Entry::first_msg_line.
	(Entry::keep_line): Filter out Signed-off-by: lines.
	(Entry::to_s): If there is no summary line and the entry body is
	empty, then keep the first line of the commit message as the
	summary.
	(process_git_log): Better detection of the summary line.  Also
	keep a handle on the first line of the commit message.

Signed-off-by: Dodji Seketeli <dodji@seketeli.org>