GitMe: check if cygwin is in PATH; if so abort installer

Johannes, Marius,
I pushed a commit to msysgit.git's mob branch that adds
a check to the GitMe installer if cygwin is in the PATH:

32ad66aed4899bab11849bdecf254a823cb6cf4b GitMe: check if cygwin is in  
PATH; if so abort installer.

I did check the shell script but I didn't build a full
installer. The change seems obvious to me. However, I
did not verified it in a real-world test.

btw,
would you like to see patches sent to the list for review?
Or is pushing them to the mob branch sufficient? Pushing
them to mob only is much easier for me.

	Steffen