Merge branch 'jk/difftool-command-not-found'

"git difftool" by default ignores the error exit from the backend
commands it spawns, because often they signal that they found
differences by exiting with a non-zero status code just like "diff"
does; the exit status codes 126 and above however are special in
that they are used to signal that the command is not executable,
does not exist, or killed by a signal.  "git difftool" has been
taught to notice these exit status codes.

* jk/difftool-command-not-found:
  difftool: always honor fatal error exit codes