swc-windows-installer.sh: Generalize msysGit download link

swc-windows-installer.sh: Generalize msysGit download link

SWC's current setup instructions use both "Git for Windows" and "Git
Bash" [1].  They link to the Git for Windows installer [2] and not the
msysGit installer [3].  Since SWC has Make lessons [4], I think it
makes more sense to have students installing msysGit (which has Make
[5]) than to have them install Git for Windows (which does not have
Make [6]).

However, none of this matters for *this* Windows installer.  Because
this script works with both the full msysGit and the minimal Git for
Windows, I'm sticking to msysGit (the more generic project name), and
generalizing the suggested link to
http://code.google.com/p/msysgit/downloads/list.

[1]: https://github.com/swcarpentry/bc/blob/5c26313b0525e9d0b911d9558592a9e33cd03c1e/_includes/setup.html#L86
[2]: http://code.google.com/p/msysgit/downloads/list?q=full+installer+official+git
[3]: http://code.google.com/p/msysgit/downloads/list?q=installer+hack+on+git
[4]: https://github.com/swcarpentry/bc/tree/master/intermediate/make
[5]: https://github.com/msysgit/msysgit#the-build-environment
[6]: https://github.com/msysgit/msysgit#the-relationship-between-msysgit-and-git-for-windows