Add support for ETag in 'plain' view

When downloading a blob identified by its path, the client might want
to know if the blob has been modified since a previous download of the
same path. To this end, an ETag containing the blob SHA1 seems to be
ideal.

Todo: add support for HEAD requests...

Suggested-by: Owen Taylor <otaylor@redhat.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-plain.c

@@ -34,6 +34,7 @@ static void print_object(const unsigned char *sha1, const char *path)
 	ctx.page.mimetype = "text/plain";
 	ctx.page.filename = fmt("%s", path);
 	ctx.page.size = size;
+	ctx.page.etag = sha1_to_hex(sha1);
 	cgit_print_http_headers(&ctx);
 	html_raw(buf, size);
 	match = 1;