magit-commit-add-log: cosmetics
magit-commit-add-log: add newline before first changelog
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.
Merge branch 'master' into next

Conflicts:
	magit.el