completion: add git config am.threeWay completion

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2343,6 +2343,7 @@ _git_config ()
 		advice.statusHints
 		alias.
 		am.keepcr
+		am.threeWay
 		apply.ignorewhitespace
 		apply.whitespace
 		branch.autosetupmerge