swc-windows-installer.py: Add install_msysgit_binary and install Make

The whole msysGit dev environment is large [1]:

  On Fri, Jun 13, 2014 at 01:22:01AM -0700, Mike Jackson wrote:
  > Disk space is cheap but 2GB is still a big ask - attendees laptops
  > can be quite old and limited in terms of space.

And may be more difficult to install than the Git for Windows
packaging [2]:

  On Fri, Aug 15, 2014 at 12:46:21PM -0700, Ethan White wrote:
  > My concern with installing msysGit is that the installer process
  > was substantially more complicated (at least the last time I
  > looked 6-12 months ago). My concern is that folks will get hung up
  > on all of the options and at best feel confused/overwhelmed and at
  > worse not end up with a working install.

Since that simple option (replace Git for Windows with the full
msysGit) isn't available, with this commit we just grab the Make
executable from the msysGit repository.  To make grabbing additional
binaries easier, I've implemented this with the generic
install_msysgit_binary.  I've also pinned the download to the most
recent tag (Git-1.9.4-preview20140815), to keep the sha1 from changing
under our feet.  However, the make.exe binary was last touched on
2012-01-26 [3] and the last hash-changing commit was on 2007-08-06
[4], so it's not exactly a high-churn target ;).

It would be nice if Windows came with a package manager (or even if
msysGit was compatible with the upstream MSYS [5]) so we didn't have
to jump through all these hoops.

[1]: https://github.com/swcarpentry/bc/pull/533#issuecomment-45986839
[2]: https://github.com/swcarpentry/windows-installer/issues/6#issuecomment-52349647
[3]: https://github.com/msysgit/msysgit/commit/cb9836b8a5ea9aa8f2cb7a373e58eeb6c000d4a7
[4]: https://github.com/msysgit/msysgit/commit/2914373028897add73217340e515f5d69d7dd027
[5]: http://thread.gmane.org/gmane.comp.version-control.git/246514/focus=20055
     Subject: Re: Re: [ANNOUNCE] WinGit - native x86/x64 Git for Windows
     From: Heiko Voigt <hvoigt@hvoigt.net>
     Date: 2014-04-19 18:42:10 GMT
     part of a thread started with [6].
[6]: http://thread.gmane.org/gmane.comp.version-control.git/245734
     Subject: [ANNOUNCE] WinGit - native x86/x64 Git for Windows
     From: Marat Radchenko <marat@slonopotamus.org>
     Date: 2014-04-03 13:18:50 GMT