diff3: allow diff3 ed scripts to generate deletions

diff3 with the -e (ed script flag) can generate line deletions, add
support for deletions and add a test case to exercise this behaviour.
This functionality was unearthed through comparison of bsd diff3 and gnu
diff3 output.

Reviewed by: pstef
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34912