bash: Add completion for git diff --base --ours --theirs

Signed-off-by: NTeemu Likonen <tlikonen@iki.fi>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -641,6 +641,7 @@ _git_diff ()
 			--ignore-all-space --exit-code --quiet --ext-diff
 			--no-ext-diff
 			--no-prefix --src-prefix= --dst-prefix=
+			--base --ours --theirs
 			"
 		return
 		;;