gitweb: add 'raw' blob_plain link in history overview

For people that work with very large plain text files it may be easier
if one can bypass viewing the htmlized blob and instead click directly
to the raw file (rather then click through 'blob' and then to 'raw').

Signed-off-by: Job Snijders <redacted>
Reviewed-by: Giuseppe Bilotta <redacted>
Signed-off-by: Junio C Hamano <redacted>