Fix issues with LFS on GitHub (SSH)

Fix issues with LFS on GitHub (SSH)

 * URIish seems to have a tiny feature (bug?). The path of the URI
   starts with a '/' only if the URI has a port set (it seems).
 * GitHub does not return SSH authorization on a single line as Gerrit
   does - need to account for that.
 * Increase the SSH git-lfs-authenticate timeout, as GitHub sometimes
   responds slower than expected.
 * Guard against NPE in case the download action does not contain any
   additional headers.

Change-Id: Icd1ead3d015479fd4b8bbd42ed42129b0abfb95c
Signed-off-by: Markus Duft <markus.duft@ssi-schaefer.com>