diff: Don't treat null characters like carriage returns in readhash().

The implicit fall-through in the !D_FORCEASCII case caused null
characters to be treated as carriage returns honoring the D_STRIPCR,
D_FOLDBLANKS, and D_IGNOREBLANKS flags.

Reported by: GCC -Wimplicit-fallthrough
Reviewed by: bapt
Fixes: 3cbf98e2bee9 diff: read whole files to determine if they are ASCII text
Differential Revision: https://reviews.freebsd.org/D36813

(cherry picked from commit 4e0771714d6192119dcf7553b921bf88866d8925)