bisect--helper: `write_terms` shell function in C

Reimplement the `write_terms` shell function in C and add a `write-terms`
subcommand to `git bisect--helper` to call it from git-bisect.sh . Also
remove the subcommand `--check-term-format` as it can now be called from
inside the function write_terms() C implementation.

Also `|| exit` is added when calling write-terms subcommand from
git-bisect.sh so as to exit whenever there is an error.

Using `--write-terms` subcommand is a temporary measure to port shell
function to C so as to use the existing test suite. As more functions
are ported, this subcommand will be retired and its implementation will
be called by some other method.

Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-bisect.sh b/git-bisect.sh
index a727c59..a80fc44 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -209,7 +209,7 @@
 	eval "$eval true" &&
 	if test $must_write_terms -eq 1
 	then
-		write_terms "$TERM_BAD" "$TERM_GOOD"
+		git bisect--helper --write-terms "$TERM_BAD" "$TERM_GOOD" || exit
 	fi &&
 	echo "git bisect start$orig_args" >>"$GIT_DIR/BISECT_LOG" || exit
 	#
@@ -557,18 +557,6 @@
 	fi
 }
 
-write_terms () {
-	TERM_BAD=$1
-	TERM_GOOD=$2
-	if test "$TERM_BAD" = "$TERM_GOOD"
-	then
-		die "$(gettext "please use two different terms")"
-	fi
-	git bisect--helper --check-term-format "$TERM_BAD" bad || exit
-	git bisect--helper --check-term-format "$TERM_GOOD" good || exit
-	printf '%s\n%s\n' "$TERM_BAD" "$TERM_GOOD" >"$GIT_DIR/BISECT_TERMS"
-}
-
 check_and_set_terms () {
 	cmd="$1"
 	case "$cmd" in
@@ -582,13 +570,17 @@
 		bad|good)
 			if ! test -s "$GIT_DIR/BISECT_TERMS"
 			then
-				write_terms bad good
+				TERM_BAD=bad
+				TERM_GOOD=good
+				git bisect--helper --write-terms "$TERM_BAD" "$TERM_GOOD" || exit
 			fi
 			;;
 		new|old)
 			if ! test -s "$GIT_DIR/BISECT_TERMS"
 			then
-				write_terms new old
+				TERM_BAD=new
+				TERM_GOOD=old
+				git bisect--helper --write-terms "$TERM_BAD" "$TERM_GOOD" || exit
 			fi
 			;;
 		esac ;;