mergetool: check return value from read

The process may not even have the standard input open in which case it
will get stuck in an infinite loop to prompt and read nothing.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -100,7 +100,7 @@ resolve_deleted_merge () {
 	else
 	    printf "Use (c)reated or (d)eleted file, or (a)bort? "
 	fi
-	read ans
+	read ans || return 1
 	case "$ans" in
 	    [mMcC]*)
 		git add -- "$MERGED"
@@ -122,7 +122,7 @@ resolve_deleted_merge () {
 resolve_submodule_merge () {
     while true; do
 	printf "Use (l)ocal or (r)emote, or (a)bort? "
-	read ans
+	read ans || return 1
 	case "$ans" in
 	    [lL]*)
 		if ! local_present; then
@@ -249,7 +249,7 @@ merge_file () {
     describe_file "$remote_mode" "remote" "$REMOTE"
     if "$prompt" = true; then
 	printf "Hit return to start merge resolution tool (%s): " "$merge_tool"
-	read ans
+	read ans || return 1
     fi
 
     if base_present; then
@@ -320,7 +320,7 @@ done
 prompt_after_failed_merge() {
     while true; do
 	printf "Continue merging other unresolved paths (y/n) ? "
-	read ans
+	read ans || return 1
 	case "$ans" in
 
 	    [yY]*)