Stop runaway git process creation under MS Windows

https://github.com/gitextensions/gitextensions/pull/1286/ tried to fix
this problem, but the solution doesn't work under MS Windows 7.

Module.IsRunningGitProcess() invokes "ps" to try to look for git
processes, but under MS Windows 7, the only two processes that are ever
seen are "/bin/sh" and "/bin/ps", i.e. the processes involved with the
"ps" request itself.  Therefore, the test fails, and if MS Visual Studio
builds in the same git repo being watched by Git Extensions, an
explosion of git processes grinds the machine to a halt quickly.

I don't know how to fix Module.IsRunningGitProcess() under MS Windows;
it seems that the existing technique isn't applicable to MS Windows, and
isn't repairable.  Until a better solution comes along, I simply
restored part of the solution that was removed by the above-referenced
pull request.  This solves the problem for me.

I've only noticed this with one particular repo, and with MS Visual
Studio 2012.  It seems like I should have seen this long ago.
Go figure.

(cherry picked from commit c1f8ee7b65019fe7596a7d71fd4ad750043d6b53)
Cover the case when update ends with error.