Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.

The index's checksum is not an object ID, so we should not use the
`git_oid` type.  Use a byte array for checksum calculation and storage.

Deprecate the `git_index_checksum` function without a replacement.  This
is an abstraction that callers should not care about (and indeed do not
seem to be using).

Remove the unused `git_index__changed_relative_to` function.

Introduce `git_fs_path`, which operates on generic filesystem paths.
`git_path` will be kept for only git-specific path functionality (for
example, checking for `.git` in a path).

libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

The static test data is erroneously initialized with a length of 0 for
three of the strings.  This means the tests are not actually examining
those strings.  Provide the length.

It was reported that, given a file "abc.txt", a diff will be shown if an
empty directory "abb/" is created, but not if "abd/" is created. Add a
test to verify that we do the right thing here and do not depend on any
ordering.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

Use the new-style index names throughout our own codebase.

Reported by Coverity, CID 1393678-1393697.

Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.

Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

When `core.symlinks = false`, we write the symlinks content (target)
to a regular file.  We should ensure that when we later see that
regular file, we treat it specially - and that changing that regular
file would actually change the symlink target.  (For compatibility
with Git for Windows).

Ensure that a diff with the workdir is not erroneously returning
directories.

Some nicer refactoring for index iteration walks.

The index iterator doesn't binary search through the pathlist space,
since it lacks directory entries, and would have to binary search
each index entry and all its parents (eg, when presented with an index
entry of `foo/bar/file.c`, you would have to look in the pathlist for
`foo/bar/file.c`, `foo/bar` and `foo`).  Since the index entries and the
pathlist are both nicely sorted, we walk the index entries in lockstep
with the pathlist like we do for other iteration/diff/merge walks.

When using literal pathspecs in diff with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`
turn on the faster iterator pathlist handling.

Updates iterator pathspecs to include directory prefixes (eg, `foo/`)
for compatibility with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`.

Document that `GIT_DIFF_PATHSPEC_DISABLE` is not necessarily about
explicit path matching, but also includes matching of directory
names.  Enforce this in a test.

Test that workdir diffs, when presented with UPDATE_INDEX, only
write the index when they actually make a change.

This fixes a bug where if a file was in conflicted state in either diff,
it would not always remain in conflicted state in the merged diff.

When a file on the workdir has the same or a newer timestamp than the
index, we need to perform a full check of the contents, as the update of
the file may have happened just after we wrote the index.

The iterator changes are such that we can reach inside the workdir
iterator from the diff, though it may be better to have an accessor
instead of moving these structs into the header.

These tests want to test that we don't recalculate entries which match
the index already. This is however something we force when truncating
racily-clean entries.

Tick the index forward as we know that we don't perform the
modifications which the racily-clean code is trying to avoid.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

A variety of data patterns for diffs verified to match the
behavior of binary detection with Git on the command line.

This is a proposed adjustment to the trace APIs.  This makes the
trace levels into a bitmask so that they can be selectively enabled
and adds a callback-level payload, plus a message-level payload.

This makes it easier for me to a GIT_TRACE_PERF callbacks that
are simply bypassed if the PERF level is not set.

This adds an option to refresh the stat cache while generating
status.  It also rips out the GIT_PERF stuff I had an makes use
of the trace API to keep statistics about what happens during diff.

When diff is scanning the working directory, if it finds a file
where it is not sure if the index entry matches the working dir,
it will recalculate the OID (which is pretty expensive).  This
adds a new flag to diff so that if the OID calculation finds that
the file actually has not changed (i.e. just the modified time was
altered or such), then it will refresh the stat cache in the index
so that future calls to diff will not have to check the oid again.

When we think the stat cache in the index seems valid and the size
or mode of a file has definitely changed, then don't bother trying
to recalculate the OID of the workdir bits to confirm that it is
modified - just accept that it is modified.

This can result in files that show as modified with no actual diff,
but the behavior actually appears to match Git on the command line.

This also includes a minor optimization to not perform a submodule
lookup on the ".git" directory itself.

When a directory containing a .git directory (or even just a plain
gitlink) was found, libgit2 was going out of its way to treat it
specially.  This seemed like it was necessary because the diff
code was not originally emulating Git's behavior for untracked
directories correctly (i.e. scanning for ignored vs untracked items
inside).  Now that libgit2 diff mimics Git's untracked directory
behavior, the special handling for contained Git repos is actually
incorrect and this commit rips it out.

It's hard or even impossible to correctly free the string buffer
allocated by git_patch_to_str in some circumstances. Drop the function
so people have to use git_patch_to_buf instead - git_buf has a dedicated
destructor.