compat/mingw: implement a native locate_in_PATH()

compat/mingw: implement a native locate_in_PATH()

since 5e1f28d (bisect--helper: reimplement `bisect_visualize()` shell
 function in C, 2021-09-13) `git bisect visualize` uses exists_in_PATH()
to check wether it should call `gitk`, but exists_in_PATH() relies on
locate_in_PATH() which currently only understands POSIX-ish PATH variables
(a list of paths, separated by colons) on native Windows executables
we encounter Windows PATH variables (a list of paths that often contain
drive letters (and thus colons), separated by semicolons). Luckily we do
already have a function that can lookup executables on windows PATHs:
path_lookup(). Implement a small replacement for the existing
locate_in_PATH() based on path_lookup().

Reported-by: Louis Strous <Louis.Strous@intellimagic.com>
Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>