t9001: avoid non-portable '\n' with sed

t9001 used a '\n' in a sed expression to split one line into two
lines, but the usage of '\n' in the "replacement string" is not
portable.

The '\n' can be used to match a newline in the "pattern space",
but otherwise the meaning of '\n' is unspecified in POSIX.

- Gnu versions of sed will treat '\n' as a newline character.
- Other versions of sed (like /usr/bin/sed under Mac OS X)
  simply ignore the '\' before the 'n', treating '\n' as 'n'.

For reference see:
pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html
http://www.gnu.org/software/sed/manual/sed.html

As the test already requires perl as a prerequisite, use perl
instead of sed.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9001-send-email.sh

@@ -1342,7 +1342,7 @@ test_cover_addresses () {
 	git format-patch --cover-letter -2 -o outdir &&
 	cover=`echo outdir/0000-*.patch` &&
 	mv $cover cover-to-edit.patch &&
-	sed "s/^From:/$header: extra@address.com\nFrom:/" cover-to-edit.patch >"$cover" &&
+	perl -pe "s/^From:/$header: extra\@address.com\nFrom:/" cover-to-edit.patch >"$cover" &&
 	git send-email \
 	  --force \
 	  --from="Example <nobody@example.com>" \