Specify Git and PuTTY locations with environment variables

In addition to Git Extensions' existing auto-detection logic it would be nice if the locations of Git and PuTTY could be specified using environment variables.

This is espescially useful in combination with a dependency-injecting package manager such as [Zero Install](http://0install.de), which uses environment variables to tell applications where they can find their dependencies.
Yes, I know that, but users are used to that some settings take effect after the app is restarted.
If it is a common case then we will be able to add a support for updating per process environment variable if it differs from the current value.