Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Our blob size is a `git_off_t`, which is a signed 64 bit int.  This may
be erroneously negative or larger than `SIZE_MAX`.  Ensure that the blob
size fits into a `size_t` before casting.

Currently, parsing objects is strictly tied to having an ODB object
available. This makes it hard to parse an object when all that is
available is its raw object and size. Furthermore, hacking around that
limitation by directly creating an ODB structure either on stack or on
heap does not really work that well due to ODB objects being reference
counted and then automatically free'd when reaching a reference count of
zero.

In some occasions parsing raw objects without touching the ODB
is actually recuired, though. One use case is for example object
verification, where we want to assure that an object is valid before
inserting it into the ODB or writing it into the git repository.

Asa first step towards that, introduce a distinction between raw and ODB
objects for blobs. Creation of ODB objects stays the same by simply
using `git_blob__parse`, but a new function `git_blob__parse_raw` has
been added that creates a blob from a pair of data and size. By setting
a new flag inside of the blob, we can now distinguish whether it is a
raw or ODB object now and treat it accordingly in several places.

Note that the blob data passed in is not being copied. Because of that,
callers need to make sure to keep it alive during the blob's life time.
This is being used to avoid unnecessarily increasing the memory
footprint when parsing largish blobs.

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

The size data in the index may not reflect the actual size of the
blob data from the ODB when content filtering comes into play.
This commit fixes rename detection to use the actual blob size when
calculating data signatures instead of the value from the index.

Because of a misunderstanding on my part, I first converted the
git_index_add_bypath API to use the post-filtered blob data size
in creating the index entry.  I backed that change out, but I
kept the overall refactoring of that routine and the new internal
git_blob__create_from_paths API because it eliminates an extra
stat() call from the code that adds a file to the index.

The existing tests actually cover this code path, at least when
running on Windows, so at this point I'm not adding new tests to
cover the changes.

This unifies the object parse functions into one signature that
takes an odb_object.

This adds create and free callback to the git_objects_table so
that more of the creation and destruction of objects can be table
driven instead of using switch statements.  This also makes the
semantics of certain object creation functions consistent so that
we can make better use of function pointers.  This also fixes a
theoretical error case where an object allocation fails and we
end up storing NULL into the cache.

This adds some initial unit tests for file filtering and fixes
some simple bugs in filter application.