The function `git_packfile_stream_free` frees all state of the packfile
stream without freeing the structure itself. This naming makes it hard
to spot whether it will try to free the pointer itself or not, causing
potential future errors. Due to this reason, we have decided to name a
function freeing state without freeing the actual struture a "dispose"
function.

Rename `git_packfile_stream_free` to `git_packfile_stream_dispose` as a
first example following this rule.

If an element has been cached, but then the call to
packfile_unpack_compressed() fails, the very next thing that happens is
that its data is freed and then the element is not removed from the
cache, which frees the data again.

This change sets obj->data to NULL to avoid the double-free. It also
stops trying to resolve deltas after two continuous failed rounds of
resolution, and adds a test for this.

The type of |base_offset| in get_delta_base() is `git_off_t`, which is a
signed `long`. That means that we need to make sure that the 8 most
significant bits are zero (instead of 7) to avoid an overflow when it is
shifted by 7 bits.

Found using libFuzzer.

Include fixups

This was pulled over from git.git, and is an experiment in
making binary-searching lists of sha1s faster. It was never
compiled by default (nor was it used upstream by default
without a special environment variable).

Unfortunately, it is actually slower in practice, and
upstream is planning to drop it in
git/git@f1068efefe6dd3beaa89484db5e2db730b094e0b (which has
some timing results). It's worth doing the same here for
simplicity.

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 `git_buf_init` function has an optional length parameter, which will
cause the buffer to be initialized and allocated in one step. This can
be used instead of static initialization with `GIT_BUF_INIT` followed by
a `git_buf_grow`. This patch does so for two functions where it is
applicable.