merge, pull: add '--(no-)log' command line option

These are the command line option equivalents of the 'merge.log' config
variable.

The patch also updates documentation and bash completion accordingly, and
adds a test.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -779,7 +779,7 @@ _git_merge ()
 		;;
 	--*)
 		__gitcomp "
-			--no-commit --no-stat --squash --strategy
+			--no-commit --no-stat --log --no-log --squash --strategy
 			"
 		return
 	esac