Add "raw" output option to blobs in "tree" view format

Add "raw" output option to blobs in "tree" view format

Add a "raw" output option to blobs in "tree" view format, so that the
user doesn't have to click on "blob", wait for the (binary) file to be
uploaded and shown in "blob" mode, and then click on "plain" to
download the (binary) file.

This is useful when the file is clearly binary and we don't want the
browser to upload and display it in "blob" mode, but we just want to
download it.  Case in point: pdf files, wlg.

Note: the "raw" format is equivalent to the blob->plain view, not
blob->head view. I.e. the view has the hash of the file as listed
by git-ls-tree, not just "HEAD".

Signed-off-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>