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.

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

When no payload is set for `crlf_apply` we try to compute the
crlf attributes ourselves with `crlf_check`. When the function
determines that the current file does not require any treatment
we return the GIT_PASSTHROUGH error code without actually
allocating the out-pointer, which indicates the file should not
be passed through the filter.

The `crlf_apply` function explicitly checks for the
GIT_PASSTHROUGH return code and ignores it. This means we will
try to apply the crlf-filter to the current file, leading us to
dereference the unallocated payload-pointer.

Fix this obviously incorrect behavior by not treating
GIT_PASSTHROUGH in any special way. This is the correct thing to
do anyway, as the code indicates that the file should not be
passed through the filter.

Perform LF->CRLF for core.autocrlf=true on non-Win32 because core
git does.

Use statistics (like core git) to control the behavior of the
to workdir CRLF filter.

For consistency with the rest of the library, where an opt is an
options *structure*.

Git for Windows 1.9.4 changed the behavior when the text=auto
attribute is specified and core.autocrlf=false.  Previous observed
behavior would *not* filter files when going into the working
directory, the new behavior *does* filter.  Update our behavior to match.