use consistent names for the #include / #define header guard pattern.

We want to do this in order to get FETCH_HEAD to be empty when we start updating
it due to fetching from the remote.

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.

When fsync'ing files, fsync the parent directory in the case where we
rename a file into place, or create a new file, to ensure that the
directory entry is flushed correctly.

Add a custom `O_FSYNC` bit (if it's not been defined by the operating
system`) so that `git_futils_writebuffer` can optionally do an `fsync`
when it's done writing.

We call `fsync` ourselves, even on systems that define `O_FSYNC` because
its definition is no guarantee of its actual support.  Mac, for
instance, defines it but doesn't support it in an `open(2)` call.

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.

git_index_entry__init_from_stat: set nsec fields in entry stats

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.