mergetool: Remove explicit references to /dev/tty

mergetool used /dev/tty to switch back to receiving input from the user
via inside a block with a redirected stdin.

This harms testability, so change mergetool to save its original stdin
to an alternative fd in this block and restore it for those sub-commands
that need the original stdin.

Includes additional compatibility fix from Jonathan Nieder.

Tested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -35,7 +35,7 @@ check_unchanged () {
 		while true; do
 			echo "$MERGED seems unchanged."
 			printf "Was the merge successful? [y/n] "
-			read answer < /dev/tty
+			read answer
 			case "$answer" in
 			y*|Y*) status=0; break ;;
 			n*|N*) status=1; break ;;

git-mergetool.sh

@@ -271,17 +271,21 @@ if test $# -eq 0 ; then
 	echo "No files need merging"
 	exit 0
     fi
+
+    # Save original stdin
+    exec 3<&0
+
     echo Merging the files: "$files"
     git ls-files -u |
     sed -e 's/^[^	]*	//' |
     sort -u |
     while IFS= read i
     do
 	if test $last_status -ne 0; then
-	    prompt_after_failed_merge < /dev/tty || exit 1
+	    prompt_after_failed_merge <&3 || exit 1
 	fi
 	printf "\n"
-	merge_file "$i" < /dev/tty > /dev/tty
+	merge_file "$i" <&3
 	last_status=$?
 	if test $last_status -ne 0; then
 	    rollup_status=1