Upstream git.git has changed the way how packfiles are named.
Previously, they were using a hash of the contained object's OIDs, which
has then been changed to use the hash of the complete packfile instead.
See 1190a1acf (pack-objects: name pack files after trailer hash,
2013-12-05) in the git.git repository for more information on this
change.
This commit changes our logic to match the behavior of core git.
This adds tests that try canceling an indexer operation from
within the progress callback.
After writing the tests, I wanted to run this under valgrind and
had a number of errors in that situation because mmap wasn't
working. I added a CMake option to force emulation of mmap and
consolidated the Amiga-specific code into that new place (so we
don't actually need separate Amiga code now, just have to turn on
-DNO_MMAP).
Additionally, I made the indexer code propagate error codes more
reliably than it used to.
