ssdiff: point line links at exact blob by prepending "id="

Previously, the id_str (i.e. the current or diffed-against commit's
SHA1 ID) was simply concatenated to the URL. Now, prepend an "id="
string so that the links actually point to the right blobs and thus
the exact lines.

Signed-off-by: Julius Plenz <plenz@cis.fu-berlin.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>