bisect--helper: `bisect_start` shell function partially in C

bisect--helper: `bisect_start` shell function partially in C

Reimplement the `bisect_start` shell function partially in C and add
`bisect-start` subcommand to `git bisect--helper` to call it from
git-bisect.sh .

The last part is not converted because it calls another shell function.
`bisect_start` shell function will be completed after the `bisect_next`
shell function is ported in C.

Using `--bisect-start` subcommand is a temporary measure to port shell
function in C so as to use the existing test suite. As more functions
are ported, this subcommand will be retired and will be called by some
other methods.

Also introduce a method `bisect_append_log_quoted` to keep things short
and crisp.

Note that we are a bit lax about command-line parsing because the helper
is not supposed to be called by the user directly (but only from the git
bisect script).

Helped-by: Ramsay Jones <redacted>
Helped-by: Stephan Beyer <redacted>
Mentored-by: Lars Schneider <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Johannes Schindelin <redacted>
Signed-off-by: Pranit Bauva <redacted>
Signed-off-by: Tanushree Tumane <redacted>
Signed-off-by: Junio C Hamano <redacted>