diff: Use start of change when searching for function

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