diff: Use start of change when searching for function

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.

PR: 262086
Reviewed by: bapt, mckusick, mhorne
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D34328