Use perl instead of sed for t8006-blame-textconv test

Use perl instead of sed for t8006-blame-textconv test

In test 'blame --textconv with local changes' of t8006-blame-textconv,
using /usr/xpg4/bin/sed (as set by SANE_TOOL_PATH), an additional
newline was added to the output from the 'helper' script.

This was noted by sed with a message such as:
sed: Missing newline at end of file zero.bin.

Sed then exits with status 2 causing the helper script to also exit
with status 2.

In turn, this was triggering a fatal error from git blame:
fatal: unable to read files to diff

To work around this difference in sed behaviour, use perl -p instead
of sed -e as it exits cleanly and does not insert the additional
newline.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>