Merge branch 'jc/index-v4'

Trivially shrinks the on-disk size of the index file to save both I/O and
checksum overhead.

The topic should give a solid base to build on further updates, with the
code refactoring in its earlier parts, and the backward compatibility
mechanism in its later parts.

* jc/index-v4:
  index-v4: document the entry format
  unpack-trees: preserve the index file version of original
  update-index: upgrade/downgrade on-disk index version
  read-cache.c: write prefix-compressed names in the index
  read-cache.c: read prefix-compressed names in index on-disk version v4
  read-cache.c: move code to copy incore to ondisk cache to a helper function
  read-cache.c: move code to copy ondisk to incore cache to a helper function
  read-cache.c: report the header version we do not understand
  read-cache.c: make create_from_disk() report number of bytes it consumed
  read-cache.c: allow unaligned mapping of the index file
  cache.h: hide on-disk index details
  varint: make it available outside the context of pack

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