Introduce --pretty=oneline format.

This introduces --pretty=oneline to git-rev-tree and
git-rev-list commands to show only the first line of the commit
message, without frills. 

Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.h

@@ -40,6 +40,7 @@ enum cmit_fmt {
 	CMIT_FMT_DEFAULT = CMIT_FMT_MEDIUM,
 	CMIT_FMT_SHORT,
 	CMIT_FMT_FULL,
+	CMIT_FMT_ONELINE,
 };
 
 extern enum cmit_fmt get_commit_format(const char *arg);