mingw: embed a manifest to trick UAC into Doing The Right Thing

On Windows >= Vista, not having an application manifest with a
requestedExecutionLevel can cause several kinds of confusing behavior.

The first and more obvious behavior is "Installer Detection" of the
"User Account Control" (also known as "UAC") feature, where Windows
sometimes decides (by looking at things like the file name and even
sequences of bytes within the executable) that an executable is an
installer and should run elevated (causing the well-known popup dialog
to appear). In Git's context, subcommands such as "git patch-id" or "git
update-index" fall prey to this behavior.

The second and more confusing behavior is "File Virtualization". It
means that when files are written without having write permission, it
does not fail (as expected), but they are instead redirected to
somewhere else. When the files are read, the original contents are
returned, though, not the ones that were just written somewhere else.
Even more confusing, not all write accesses are redirected; Trying to
write to write-protected .exe files, for example, will fail instead of
redirecting.

In addition to being unwanted behavior, File Virtualization causes
dramatic slowdowns in Git (see for instance
http://code.google.com/p/msysgit/issues/detail?id=320).

A third unwanted behavior of Windows >= Vista is that it lies about the
Windows version when calling `GetWindowsVersionEx()`.

There are two ways to prevent these unwanted behaviors: Either you embed
an application manifest (which really is an XML document conforming to a
specific schema) within all your executables, or you add an external
manifest (a file with the same name followed by `.manifest`) to all your
executables. Since Git's builtins are hardlinked (or copied), it is
simpler and more robust to embed a manifest.

Recent enough MSVC compilers already embed a working internal manifest,
and building with mingw-w64 (which is the case in Git for Windows' SDK)
does it, too, but for MinGW you have to do so by hand.

In any case, it is better to be explicit about this manifest, that way
changes in the compiler toolchain won't surprise us (as mingw-w64 once
did when it broke `GetWindowsVersionEx()` by mistake).

References:
  - New UAC Technologies for Windows Vista
    http://msdn.microsoft.com/en-us/library/bb756960.aspx
  - Create and Embed an Application Manifest (UAC)
    http://msdn.microsoft.com/en-us/library/bb756929.aspx

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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