bisect: Store first bad commit as comment in log file

When bisect successfully finds a single revision, the first bad commit
should be shown to human readers of 'git bisect log'.

This resolves the apparent disconnect between the bisection result and
the log when a bug reporter says "I know that the first bad commit is
$rev, as you can see from $(git bisect log)".

Signed-off-by: Torstein Hegge <hegge@resisty.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hegge

git-bisect.sh

@@ -311,7 +311,13 @@ bisect_next() {
 	res=$?
 
 	# Check if we should exit because bisection is finished
-	test $res -eq 10 && exit 0
+	if test $res -eq 10
+	then
+		bad_rev=$(git show-ref --hash --verify refs/bisect/bad)
+		bad_commit=$(git show-branch $bad_rev)
+		echo "# first bad commit: $bad_commit" >>"$GIT_DIR/BISECT_LOG"
+		exit 0
+	fi
 
 	# Check for an error in the bisection process
 	test $res -ne 0 && exit $res

t/t6030-bisect-porcelain.sh

@@ -741,4 +741,22 @@ test_expect_success 'bisect: demonstrate identification of damage boundary' "
 	git bisect reset
 "
 
+cat > expected.bisect-log <<EOF
+# bad: [32a594a3fdac2d57cf6d02987e30eec68511498c] Add <4: Ciao for now> into <hello>.
+# good: [7b7f204a749c3125d5224ed61ea2ae1187ad046f] Add <2: A new day for git> into <hello>.
+git bisect start '32a594a3fdac2d57cf6d02987e30eec68511498c' '7b7f204a749c3125d5224ed61ea2ae1187ad046f'
+# good: [3de952f2416b6084f557ec417709eac740c6818c] Add <3: Another new day for git> into <hello>.
+git bisect good 3de952f2416b6084f557ec417709eac740c6818c
+# first bad commit: [32a594a3fdac2d57cf6d02987e30eec68511498c] Add <4: Ciao for now> into <hello>.
+EOF
+
+test_expect_success 'bisect log: successfull result' '
+	git bisect reset &&
+	git bisect start $HASH4 $HASH2 &&
+	git bisect good &&
+	git bisect log >bisect-log.txt &&
+	test_cmp expected.bisect-log bisect-log.txt &&
+	git bisect reset
+'
+
 test_done