bisect: add support for bisecting bare repositories

This enhances the support for bisecting history in bare repositories.

The "git bisect" command no longer needs to be run inside a repository
with a working tree; it defaults to --no-checkout when run in a bare
repository.

Two tests are included to demonstrate this behaviour.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git.c b/git.c
index 8828c18..7fdcab2 100644
--- a/git.c
+++ b/git.c
@@ -320,7 +320,7 @@
 		{ "annotate", cmd_annotate, RUN_SETUP },
 		{ "apply", cmd_apply, RUN_SETUP_GENTLY },
 		{ "archive", cmd_archive },
-		{ "bisect--helper", cmd_bisect__helper, RUN_SETUP | NEED_WORK_TREE },
+		{ "bisect--helper", cmd_bisect__helper, RUN_SETUP },
 		{ "blame", cmd_blame, RUN_SETUP },
 		{ "branch", cmd_branch, RUN_SETUP },
 		{ "bundle", cmd_bundle, RUN_SETUP_GENTLY },