bisect: treat BISECT_HEAD as a pseudo ref

Both the git-bisect.sh as bisect--helper inspected the file system
directly.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/bisect--helper.c

@@ -13,7 +13,6 @@ static GIT_PATH_FUNC(git_path_bisect_terms, "BISECT_TERMS")
 static GIT_PATH_FUNC(git_path_bisect_expected_rev, "BISECT_EXPECTED_REV")
 static GIT_PATH_FUNC(git_path_bisect_ancestors_ok, "BISECT_ANCESTORS_OK")
 static GIT_PATH_FUNC(git_path_bisect_start, "BISECT_START")
-static GIT_PATH_FUNC(git_path_bisect_head, "BISECT_HEAD")
 static GIT_PATH_FUNC(git_path_bisect_log, "BISECT_LOG")
 static GIT_PATH_FUNC(git_path_head_name, "head-name")
 static GIT_PATH_FUNC(git_path_bisect_names, "BISECT_NAMES")
@@ -164,7 +163,7 @@ static int bisect_reset(const char *commit)
 		strbuf_addstr(&branch, commit);
 	}
 
-	if (!file_exists(git_path_bisect_head())) {
+	if (!ref_exists("BISECT_HEAD")) {
 		struct argv_array argv = ARGV_ARRAY_INIT;
 
 		argv_array_pushl(&argv, "checkout", branch.buf, "--", NULL);

git-bisect.sh

@@ -41,7 +41,7 @@ TERM_GOOD=good
 
 bisect_head()
 {
-	if test -f "$GIT_DIR/BISECT_HEAD"
+	if git rev-parse --verify -q BISECT_HEAD > /dev/null
 	then
 		echo BISECT_HEAD
 	else
@@ -153,7 +153,7 @@ bisect_next() {
 	git bisect--helper --bisect-next-check $TERM_GOOD $TERM_BAD $TERM_GOOD|| exit
 
 	# Perform all bisection computation, display and checkout
-	git bisect--helper --next-all $(test -f "$GIT_DIR/BISECT_HEAD" && echo --no-checkout)
+	git bisect--helper --next-all $(git rev-parse --verify -q BISECT_HEAD > /dev/null && echo --no-checkout)
 	res=$?
 
 	# Check if we should exit because bisection is finished