completion: return the index of found word from __git_find_on_cmdline()

When using the __git_find_on_cmdline() helper function so far we've
only been interested in which one of a set of words appear on the
command line.  To complete options for some of 'git worktree's
subcommands in the following patches we'll need not only that, but the
index of that word on the command line as well.

Extend __git_find_on_cmdline() to optionally show the index of the
found word on the command line (IOW in the $words array) when the
'--show-idx' option is given.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1069,18 +1069,32 @@ __git_aliased_command ()
 	done
 }
 
-# __git_find_on_cmdline requires 1 argument
 # Check whether one of the given words is present on the command line,
 # and print the first word found.
+#
+# Usage: __git_find_on_cmdline [<option>]... "<wordlist>"
+# --show-idx: Optionally show the index of the found word in the $words array.
 __git_find_on_cmdline ()
 {
-	local word c=1
+	local word c=1 show_idx
+
+	while test $# -gt 1; do
+		case "$1" in
+		--show-idx)	show_idx=y ;;
+		*)		return 1 ;;
+		esac
+		shift
+	done
 	local wordlist="$1"
 
 	while [ $c -lt $cword ]; do
 		for word in $wordlist; do
 			if [ "$word" = "${words[c]}" ]; then
-				echo "$word"
+				if [ -n "$show_idx" ]; then
+					echo "$c $word"
+				else
+					echo "$word"
+				fi
 				return
 			fi
 		done