Remove a duplicate --not option in bash completion

This was just me being silly; I put the --not option into the
completion list twice.  There's no duplicates shown in the shell
as the shell removes them before showing them to the user.  But we
really don't need the duplicates in the source script either.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -579,7 +579,7 @@ _git_log ()
 		__gitcomp "
 			--max-count= --max-age= --since= --after=
 			--min-age= --before= --until=
-			--root --not --topo-order --date-order
+			--root --topo-order --date-order
 			--no-merges
 			--abbrev-commit --abbrev=
 			--relative-date