Eliminate confusing "won't bisect on seeked tree" failure

This error message is very confusing---it doesn't tell the user
anything about how to fix the situation. And the actual fix
for the situation ("git bisect reset") does a checkout of a
potentially random branch, (compared to what the user wants to
be on for the bisect she is starting).

The simplest way to eliminate the confusion is to just make
"git bisect start" do the cleanup itself. There's no significant
loss of safety here since we already have a general safety in
the form of the reflog.

Note: We preserve the warning for any cogito users. We do this
by switching from .git/head-name to .git/BISECT_START for the
extra state, (which is a more descriptive name anyway).

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -67,16 +67,18 @@ bisect_start() {
 	die "Bad HEAD - I need a HEAD"
 	case "$head" in
 	refs/heads/bisect)
-		if [ -s "$GIT_DIR/head-name" ]; then
-		    branch=`cat "$GIT_DIR/head-name"`
+		if [ -s "$GIT_DIR/BISECT_START" ]; then
+		    branch=`cat "$GIT_DIR/BISECT_START"`
 		else
 		    branch=master
 		fi
 		git checkout $branch || exit
 		;;
 	refs/heads/*|$_x40)
+		# This error message should only be triggered by cogito usage,
+		# and cogito users should understand it relates to cg-seek.
 		[ -s "$GIT_DIR/head-name" ] && die "won't bisect on seeked tree"
-		echo "${head#refs/heads/}" >"$GIT_DIR/head-name"
+		echo "${head#refs/heads/}" >"$GIT_DIR/BISECT_START"
 		;;
 	*)
 		die "Bad HEAD - strange symbolic ref"
@@ -353,8 +355,8 @@ bisect_reset() {
 		return
 	}
 	case "$#" in
-	0) if [ -s "$GIT_DIR/head-name" ]; then
-	       branch=`cat "$GIT_DIR/head-name"`
+	0) if [ -s "$GIT_DIR/BISECT_START" ]; then
+	       branch=`cat "$GIT_DIR/BISECT_START"`
 	   else
 	       branch=master
 	   fi ;;
@@ -365,7 +367,9 @@ bisect_reset() {
 	    usage ;;
 	esac
 	if git checkout "$branch"; then
+		# Cleanup head-name if it got left by an old version of git-bisect
 		rm -f "$GIT_DIR/head-name"
+		rm -f "$GIT_DIR/BISECT_START"
 		bisect_clean_state
 	fi
 }

t/t6030-bisect-porcelain.sh

@@ -260,7 +260,7 @@ test_expect_success 'bisect starting with a detached HEAD' '
 	git checkout master^ &&
 	HEAD=$(git rev-parse --verify HEAD) &&
 	git bisect start &&
-	test $HEAD = $(cat .git/head-name) &&
+	test $HEAD = $(cat .git/BISECT_START) &&
 	git bisect reset &&
 	test $HEAD = $(git rev-parse --verify HEAD)