stash show: use stash.showIncludeUntracked even when diff options given

stash show: use stash.showIncludeUntracked even when diff options given

If options pertaining to how the diff is displayed is provided to
`git stash show`, the command will ignore the stash.showIncludeUntracked
configuration variable, defaulting to not showing any untracked files.
This is unintuitive behaviour since the format of the diff output and
whether or not to display untracked files are orthogonal.

Use stash.showIncludeUntracked even when diff options are given. Of
course, this is still overridable via the command-line options.

Update the documentation to explicitly say which configuration variables
will be overridden when a diff options are given.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>