bisect--helper: `bisect_clean_state` shell function in C
Reimplement `bisect_clean_state` shell function in C and add a
`bisect-clean-state` subcommand to `git bisect--helper` to call it from
git-bisect.sh .
Using `--bisect-clean-state` subcommand is a measure to port shell
function to C so as to use the existing test suite. As more functions
are ported, this subcommand will be retired but its implementation will
be called by bisect_reset() and bisect_start().
Also introduce a function `mark_for_removal` to store the refs which
need to be removed while iterating through the refs.
Mentored-by: Lars Schneider <larsxschneider@gmail.com> Mentored-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>