When you install msysgit, you are asked in one of the dialogs how your PATH should be modified.

1. Use Git Bash Only
2. Run Git from the Windows Command Prompt
3. Run Git and included Unix tools from the Windows Command Prompt

If you select option 2. (which I guess most users will pick, as option 3. has a scary warning), the "<...>\Git\cmd" directory will be added to the PATH.
If you pick option 3. the "<...>\Git\bin" directory will also be added.

Now if you run "git --version" from the commandline, it will run "<...>\Git\cmd\git.cmd", which will call "git.exe --version".
I guess Stash tries to run "git.exe --version" to determine the installed git version.
If you've installed msysgit with option 3. you're fine, as "<...>\Git\bin\git.exe" is in your path.
Did you select option 2. you're out of luck as that directory as not added to your path.

The workaround for the user is to either run the msysgit installer again and select option 3. to have your PATH properly set, or edit your PATH manually.

If Stash is indeed calling "git.exe --version", this problem will be solved if Stash calls "git --version" instead.