Avoid invoking diff drivers during git-stash

Avoid invoking diff drivers during git-stash

git-stash needs to restrict itself to plumbing when running automated
diffs as part of its operation as the user may have configured a
custom diff driver that opens an interactive UI for certain/all
files.  Doing that during scripted actions is very unfriendly to
the end-user and may cause git-stash to fail to work.

Reported by Johannes Sixt

Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>