bisect: display first bad commit without forking a new process

bisect: display first bad commit without forking a new process

Previously "git diff-tree --pretty COMMIT" was run using
"run_command_v_opt" to display information about the first bad
commit.

The goal of this patch is to avoid a "fork" and an "exec" call
when displaying that information.

To do that, we manually setup revision information as
"git diff-tree --pretty" would do it, and then use the
"log_tree_commit" function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>