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>

cgit.h

@@ -180,6 +180,7 @@ struct cgit_page {
 	char *mimetype;
 	char *charset;
 	char *filename;
+	char *etag;
 	char *title;
 };