stash: partial stash specific files

`git_strarray_copy` is deprecated (and has been included in
`deprecated.h` for some time). It should not have remained in
the public `strarray.h`. Remove it.

SHA256: more SHA256 support

This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

Teach the packfile machinery to cope with SHA256.

Teach the repository about the `objectformat` extension, supporting
`sha1` always and `sha256` when the experimental sha256 support is
active.

Some callers -- like our test suite and the test suites of our language
bindings -- want to isolate the home directory to avoid accidentally
including the executing user's actual home directory data.

Previously, we combined the notion of a home directory and global
configuration -- now that this is separated, we provide users the
ability to configure both.