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.

Set a message when we fail to lock.

Also make the put function void, since it's called from free, which
cannot report errors. The only errors we can experience here are
internal state corruption, so we assert that we are trying to put a
pack which we have previously got.

Opening the same repository multiple times will currently open the same
file multiple times, as well as map the same region of the file multiple
times. This is not necessary, as the packfile data is immutable.

Instead of opening and closing packfiles directly, introduce an
indirection and allocate packfiles globally. This does mean locking on
each packfile open, but we already use this lock for the global mwindow
list so it doesn't introduce a new contention point.

As a function that appears to only be called on error paths, I don't
think it makes sense for it to return an error, or clobber the global
giterr. Note that no existing callsites actually check the return
code.

In my own application, there are errors where the real error ends
up being hidden, as git_mwindow_file_deregister() clobbers the
global giterr. I'm not sure this error is even relevant?

Up to now, the idea was that the user would do all the operations for
one repository in the same thread. Thus we could have the
memory-mapped window information thread-local and avoid any locking.

This is not practical in a few environments, such as Apple's GCD which
allocates threads arbitrarily or the .NET CLR, where the OS-level
thread can change at any moment.

Make the control structure global and protect it with a mutex so we
don't depend on the thread currently executing the code.

Once a file is registered, there is no way to deregister it, even
after the structure that contains it is no longer needed and has been
freed. This may be the source of #624.

Allow and use the deregister function to remove our file from the
global list.

This converts blob.c, fileops.c, and all of the win32 files.
Also, various minor cleanups throughout the code.  Plus, in
testing the win32 build, I cleaned up a bunch (although not
all) of the warnings with the 64-bit build.

Signed-off-by: schu <schu-github@schulog.org>