Mk/Scripts/smart_makepatch.sh: Force diff to produce the patch file instead of "Binary files ... differ" message

It helps avoid generating wrong patches, e.g. patch-grpc.gemspec in r567838 [1].

from diff manpage:

-a --text
    Treat all files as ASCII text.  Normally diff will simply print

"Binary files ... differ" if files contain binary characters.

Use of this option forces diff to produce a diff.

[1] https://cgit.FreeBSD.org/ports/diff/net/rubygem-grpc130/files/patch-grpc.gemspec?id=073c1c710a0c7254820adab9920c1903ace83fae

PR: 257027
Approved by: mat (portmgr)