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.

The only function that is named `issomething` (without underscore) was
`git_oid_iszero`.  Rename it to `git_oid_is_zero` for consistency with
the rest of the library.

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

CID 1386176, 1386177, 1388219

Use the new object_type enumeration names within the codebase.

The function `git_diff_find_similar` keeps a function of cache
similarity metrics signatures, whose size depends on the number of
deltas passed in via the `diff` parameter. In case where the diff is
empty and thus doesn't have any deltas at all, we may end up allocating
this cache via a call to `git__calloc(0, sizeof(void *))`. At least on
AIX, allocating 0 bytes will result in a `NULL` pointer being returned,
which causes us to erroneously return an OOM error.

Fix this situation by simply returning early in case where we are being
passed an empty diff, as we cannot find any similarities in that case
anyway.

A rewritten file can either be classified as a modification of its
contents or of a delete of the complete file followed by an addition of
the new content. This distinction becomes important when we want to
detect renames for rewrites. Given a scenario where a file "a" has been
deleted and another file "b" has been renamed to "a", this should be
detected as a deletion of "a" followed by a rename of "a" -> "b". Thus,
splitting of the original rewrite into a delete/add pair is important
here.

This splitting is represented by a flag we can set at the current delta.
While the flag is already being set in case we want to break rewrites,
we do not do so in case where the `GIT_DIFF_FIND_RENAMES_FROM_REWRITES`
flag is set. This can trigger an assert when we try to match the source
and target deltas.

Fix the issue by setting the `GIT_DIFF_FLAG__TO_SPLIT` flag at the delta
when it is a rename target and `GIT_DIFF_FIND_RENAMES_FROM_REWRITES` is
set.

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.