Read GITEXT_GIT environment variable
Read GITEXT_PUTTY environment variable
Tweaked CheckIfFileIsInPath()

The per-user and machine-wide PATHs are concatenated by the OS automatically, no need to do it manually.
The manual concatenation also had the unintended side-effect of ignoring per-process PATH entries.
Merge pull request #2367 from bastianeicher/master

Specify Git and PuTTY locations with environment variables