stash: fix "--staged" with binary files

stash: fix "--staged" with binary files

"git stash --staged" errors out when given binary files, after saving the
stash.

This behaviour dates back to the addition of the feature in 41a28eb6c1
(stash: implement '--staged' option for 'push' and 'save', 2021-10-18).
Adding the "--binary" option of "diff-tree" fixes this. The "diff-tree" call
in stash_patch() also omits "--binary", but that is fine since binary files
cannot be selected interactively.

Helped-By: Jeff King <redacted>
Helped-By: Randall S. Becker <redacted>
Signed-off-by: Adam Johnson <redacted>
Signed-off-by: Junio C Hamano <redacted>