Don't compute the sha-1 in `git_futils_readbuffer_updated` unless the
checksum was requested.  This means that `git_futils_readbuffer` will
not calculate the checksum unnecessarily.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

We need to save the errno, lest we clobber it in the giterr_set()
call. Also add code for reporting that a path component is missing,
which is a distinct failure mode.

Since writing multiple objects may all already exist in a single
packfile, avoid freshening that packfile repeatedly in a tight loop.
Instead, only freshen pack files every 2 seconds.

When writing an object, we calculate its OID and see if it exists in the
object database.  If it does, we need to freshen the file that contains
it.

Android NDK does not have a `struct timespec` in its `struct stat`
for nanosecond support, instead it has a single nanosecond member inside
the struct stat itself.  We will use that and use a macro to expand to
the `st_mtim` / `st_mtimespec` definition on other systems (much like
the existing `st_mtime` backcompat definition).

This reduces the chances of a crash in the thread tests. This shouldn't
affect general usage too much, since the main usage of these functions
are to read into an empty buffer.

Instead of relying on the size and timestamp, which can hide changes
performed in the same second, hash the file content's when we care about
detecting changes.

`git_futils_mkdir` does not blindly call `git_futils_mkdir_relative`.

`git_futils_mkdir_relative` is used when you have some base directory
and want to create some path inside of it, potentially removing blocking
symlinks and files in the process.  This is not suitable for a general
recursive mkdir within the filesystem.

Instead, when `mkdir` is being recursive, locate the first existent
parent directory and use that as the base for `mkdir_relative`.

Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

We set an error if we get an error when reading, but we don't bother
setting an error message for write failing. This causes a cryptic error
to be shown to the user when the target filesystem is full.

Now that `git_path_direach` lets us specify an error message to report,
set an appropriate error message while linking.

Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Ensure that the given length to `p_read` is of ssize_t and ensure
that callers test the return as if it were an `ssize_t`.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

Always do a time_t cast on st_mtime. st_mtime on Android is not
the type of time_t but has the same meaning which is the number
of seconds past epoch.

Don't try to strip trailing paths from the root directory on
Windows (trying to create `C:` will fail).

On case insensitive filesystems, we may have files in the working
directory that case fold to a name we want to write.  Remove those
files (by default) so that we will not end up with a filename that
has the unexpected case.

Walk up the tree to mkdir, which is less immediately efficient,
but allows us to look at intermediate directories that may need
attention.

Checkout can now provide performance data about the number of (some)
syscalls performed using an optional callback.

Our mkdir helper was failing is a parent directory was not
accessible even if the child directory could be created.
This changes the helper to keep trying child directories
even when the parent is unwritable.

When passed the LINK_FILES flag, the recursive copy will hardlink files
instead of copying them.

The checks to see if files were out of date in the attibute cache
was wrong because the cache-breaker data wasn't getting stored
correctly.  Additionally, when the cache-breaker triggered, the
old file data was being leaked.

This is a big refactoring of the attribute file cache to be a bit
simpler which in turn makes it easier to enforce a lock around any
updates to the cache so that it can be used in a threaded env.
Tons of changes to the attributes and ignores code.

This renames git_vector_free_all to the better git_vector_free_deep
and also contains a couple of memory leak fixes based on valgrind
checks.  The fixes are specifically: failure to free global dir
path variables when not compiled with threading on and failure to
free filters from the filter registry that had not be initialized
fully.