Git bug report: git doesn't change working directory

I'm using Windows 10. Before I install git 2.6.1.windows.1, I have 
installed git 1.9.5.github.0 (by installing GitHub Desktop), it works just 
fine.
But when I installed git 2.6.1.windows.1 (from git-scm.com), I'm not able 
to use git anymore:
 - The powershell console displayed [(unknown)] instead of [master], even 
when I changed working directory to my project, it still display 
[(unknown)]
 - When I "git add" (or "git commit", "git push"), It told me an error: 
"fatal: Not a git repository: 'C:\Program Files\Git'"
After that, I returned to the old version of git - git 1.9.5.github.0, but 
the fault still there.
I think git didn't change its working directory, I have no idea.
Now, I'm not able to work with git.
Please response me as soon as possible, thanks.