git.py: recover more elegantly if a MIDX file has the wrong version.
git.py: recover more elegantly if a MIDX file has the wrong version.

Previously we'd throw an assertion for any too-new-format MIDX file, which
isn't so good.  Let's recover more politely (and just ignore the file in
question) if that happens.

Noticed by Zoran Zaric who was testing my midx3 branch.

Signed-off-by: Avery Pennarun <e03ccfb89d416c4697ad6970ce86a92c1c28a78e@gmail.com>

ToxicFrog/bup,pombredanne/bup,apenwarr/bup,jbaber/bup,gevaerts/bup,ToxicFrog/bup,pombredanne/bup,jbaber/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,pombredanne/bup,apenwarr/bup,mhoeher/bup,tjanez/bup,mhoeher/bup,gevaerts/bup,tjanez/bup,jbaber/bup,apenwarr/bup,apenwarr/bup,tjanez/bup,mhoeher/bup,gevaerts/bup,gevaerts/bup,pombredanne/bup,ToxicFrog/bup,ToxicFrog/bup,mhoeher/bup,tjanez/bup,jbaber/bup,tjanez/bup,ToxicFrog/bup,jbaber/bup,jbaber/bup