Fix #230 : Don't spawn git status if the previous one is still running

Hello
When we are low on cpu (e.g. compiling), while working on big repos, git status may take too long and GitExtensions would spawn lots of git status, increasing the pressure on the cpu.
In the code, ps was used but is unable to detect git.exe processes.
So, in Windows context, I use Process.GetProcessesByName.
I validated my solution : problem disappears with my fix.
ded'