magit-git-executable: use bin/git.exe on Windows
magit-gitk-executable: locate executable at load-time

Locate the gitk executable at load-time when setting the default
value of `magit-gitk-executable', instead of doing it every time
`magit-run-gitk' is used.  This builds on a similar hack used
when setting the default value of `magit-git-executable'.
magit-run-{git-gui{,-blame},gitk}: use call-process

Start the subprocess using `call-process' with argument DESTINATION
being 0.  I.e. start the subprocess and immediately return 0 without
caring what happens to the subprocess afterwards.  Previously we used
`start-file-process' but that had the disadvantage that the process
object still existed when the subprocess was still running when Emacs
was being quit, needlessly forcing the termination of the subprocess.