completion: add GIT_COMPLETION_SHOW_ALL env var

When set to 1, GIT_COMPLETION_SHOW_ALL causes --git-completion-helper-all
to be passed instead of --git-completion-helper.

Signed-off-by: Ryan Zoeller <rtzoeller@rtzoeller.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 700d44a..9147fba 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -39,6 +39,11 @@
 #     When set to "1", do not include "DWIM" suggestions in git-checkout
 #     and git-switch completion (e.g., completing "foo" when "origin/foo"
 #     exists).
+#
+#   GIT_COMPLETION_SHOW_ALL
+#
+#     When set to "1" suggest all options, including options which are
+#     typically hidden (e.g. '--allow-empty' for 'git commit').
 
 case "$COMP_WORDBREAKS" in
 *:*) : great ;;
@@ -411,10 +416,17 @@
 	local options
 	eval "options=\${$var-}"
 
+	local completion_helper
+	if [ "$GIT_COMPLETION_SHOW_ALL" = "1" ]; then
+		completion_helper="--git-completion-helper-all"
+	else
+		completion_helper="--git-completion-helper"
+	fi
+
 	if [ -z "$options" ]; then
 		# leading and trailing spaces are significant to make
 		# option removal work correctly.
-		options=" $incl $(__git ${cmd/_/ } --git-completion-helper) " || return
+		options=" $incl $(__git ${cmd/_/ } $completion_helper) " || return
 
 		for i in $excl; do
 			options="${options/ $i / }"