magit-git{,k}-executable: store full path, not just name

This causes the full path to be displayed in *magit-process*, which is
a good thing, as it will help debugging windows problems.  If someone
complains about that we can still add an option to only display the
basename.
add magit-quote-curly-braces

This fixes issue #816 while keeping #757 in mind: on Windows curly
braces sometimes have to be quoted sometimes not.
magit-run-gitk: on Windows: fix putting sh on path

Also cleanup a bit.
Merge branch 'fix-windows'