t8001/t8002: blame: add empty file & partial-line tests

t8001/t8002: blame: add empty file & partial-line tests

Add boundary case tests, with and without -L, for empty file; file with
one partial line; file with one full line.

The empty file test without -L is of particular interest. Historically,
this case has been supported (empty blame output) and this test protects
against regression by a subsequent patch fixing an off-by-one bug which
incorrectly accepts -LX where X is one past end-of-file.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>