system.git.executable fails to detect git in PATH or cygwin

On Windows the detection of git from system.git.executable is failing to find git even if this is in PATH or in cygwin directory.

The detection algorithm mush be change so it would also look for PATH and for the default cygwin location C:\cygwin\bin\git.exe

Using Cygwin under Windows is a very common solution for cross-platform build systems.