Fix git-instaweb breakage on MacOS X due to the limited sed functionality

Fix git-instaweb breakage on MacOS X due to the limited sed functionality

git-instaweb relied on a pipe in a sed script, but this is not supported
by MacOS X sed when using BREs.  git-instaweb relies on a working perl
in any case, and perl re are more consistent between platforms, so
replace sed invocation with an equivalent perl invocation.

Also, fix the documented -b "" to work without giving a spurious 'command
not found' error.

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>