diff: Use change start when searching for function names

[thj]

Summary

Use the start of change when searching for a function rather than the
start of the context. In short functions if this could result in search
for the function name starting from before the function definition.

This was exposed by
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=262086

Test Plan

This fixes the previously failing diff functioname test, all other tests pass.

[mckusick]

minor format question. Change looks correct.

usr.bin/diff/diffreg.c

1566

why add the extra blank line?

[mhorne]

usr.bin/diff/diffreg.c

1465