xsa: Patch de-duping, first take

xsa: Patch de-duping, first take

First take of patch deduping: Run a separate command which, for each
version of an XSA:
 - Makes a branch with the current recipe
 - Takes the hash of that tree
 - Tries to make a branch with a potential "de-dupped" recipe
 - If that works, compare the hash with the previous hash

If the dedupped recipe applies, and the hashes match, change to using
the de-dupped recipe.  Otherwise, use the current recipe.

This will need to be reworked somewhat to support automatic de-dupping
during patch creation, but get a working copy in the tree.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>