Fix the following warning from gcc 4.2 in usr.bin/diff:

usr.bin/diff/diffreg.c: In function 'change':
usr.bin/diff/diffreg.c:1085: warning: 'i' may be used uninitialized in this function

This version of gcc is not smart enough to see that 'i' cannot actually
be used unitialized. However, the variable is confusingly re-used, so
it is better to give it another name, and clearly initialize it before
attempting to use it.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D10484