sha256: refactoring in preparation for sha256

Validate repository directory ownership

Introduce the `GIT_OPT_SET_OWNER_VALIDATION` option, so that users can
disable repository ownership validation.

Obey the `safe.directory` configuration variable if it is set in the
global or system configuration. (Do not try to load this from the
repository configuration - to avoid malicious repositories that then
mark themselves as safe.)

Pull the global configuration loader out of the symlink check so that it
can be re-used.

Test that we prevent opening directories that are not owned by
ourselves.

Provide a mock for file ownership for testability.

Ensure that the repository directory is owned by the current user; this
prevents us from opening configuration files that may have been created
by an attacker.

Provide individual file ownership checks for both the current user and
the system user, as well as a combined current user and system user
check.

The indexer expects raw oid data, provide it.

A multi-pack index uses raw oid data, use a byte array to index
into them.

The index contains entries with raw oid data, use a byte array for the
raw entry data.

A packfile contains arrays of raw oid data, use a byte array to index
into them.

The commit graph contains arrays of raw oid data, use a byte array to
index into them.

Now that oids are type-aware, they use their type to understand how many
bytes to copy.  Some callers may need to copy the raw bytes of the
object id.

This is equivalent to a memcpy that is a little more semantic.

We will talk about "raw" oids as untyped blobs of data; use a name for
the comparison function that is in keeping with that.

Create an object id from raw data instead of casting.

Don't write the object id structure, write its raw oid data.

We explicitly want to write on the id data, not the beginning of the
object data, which may contain other information in the future.

We explicitly want to hash on the id data, not the beginning of the
object data, which may contain other information in the future.

Don't assume that a `git_oid` is a particular size; allocate
`sizeof(git_oid)` instead.

A tree entry previously pointed directly into the object id within the
tree object itself; this is useful to avoid any unnecessary memory copy
(and an unnecessary use of 40 bytes per tree entry) but difficult if we
change the underlying `git_oid` object to not simply be a raw object id
but have additional structure.

This commit moves the `git_oid` directly into the tree entry; this
simplifies the tree entry creation from user data.  We now copy the
`git_oid` into place when parsing.

midx: Fix an undefined behavior (left-shift signed overflow)

Fix crash when regenerating a patch with unquoted spaces in filename

tests: support flaky stat

The 32-bit ARM QEMU builds are flaky when running `lstat`. Disable those
testing `lstat`'s `st_size` temporarily.

diff: don't stat empty file on arm32 (flaky test)

sha256: support dynamically loaded openssl

There was a missing check to ensure that the `off64_t` (which is a
signed value) didn't overflow when parsing it from the midx file. This
shouldn't have huge repercusions since the parsed value is immediately
validated afterwards, but then again, there is no such thing as "benign"
undefined behavior.

This change makes all the bitwise arithmetic happen with unsigned types
and is only casted to `off64_t` until the very end.

Thanks to Taotao Gu for finding and reporting this!

fetch: support OID refspec without dst

Our CI test infrastructure virtualizes arm32 in docker, which is a
sometimes imperfect situation. In `diff::workdir::can_diff_empty_file`,
avoid the stat to ensure that the file is zero bytes; there is an odd
issue running in qemu when emulating arm32 that we should skip.

SHA256: add a SHA256 implementation backend

gcc (mingw) warns when you cast the result of `GetProcAddress`; cast the
results to `void *` before casting them to the actual result.

When GIT_SHA1_WIN32 or GIT_SHA256_WIN32 is used, ensure that we test
both CryptoNG ("cng") and CryptoAPI.

Adding SHA256 support prompted an overdue refactoring of some of the
unnecessary complexity around the CNG/CryptoAPI abstraction.