magit-git-executable: use executable-find when initializing value

This hopefully give a slight performance bust.

The downside is that the headers in the process buffer get longer
and that the "interesting parts" might end up being truncated.

Still I hesitate to add an option to hide the full path and the
`magit-git-standard-options'; when instructing users to provide
the information required to debug some issue there would be one
more thing that we would have to make sure they actually did.

If `executable-find' fails to find Git then fallback to "git".
magit-git-executable: use git.exe on Windows

On Windows, explicitly use the "git.exe" executable instead of "git".
If that fails accept any "git".

It has been reported that this gives a considerable speedup.  I don't
know Windows but it appears that when asking for a "foo" one might get
a "foo.exe" or a "foo.cmd" or a "foo".  It also appears that "git.cmd"
is earlier on the `exec-path' than "git.exe", or that the former is
preferred for some other reason.
Merge branch 'git-executable'