should improve docs for windows manual installation with Git-Bash

[datphan]

Only happen when we use cygwin

• So it can cover: Issues with shell when SSH-ing into Vagrant hashicorp/vagrant#9143 (comment)

• As well as win 10 pro
  

Rsync with Git Bash

To use rsync with Git Bash:
download rsync-3.1 at http://www2.futureware.at/~nickoe/msys2-mirror/msys/x86_64/

extract and copy rsync.exe to C:\Program Files\Git\usr\bin

[hoatle]

so we're going to support Git Bash, make sure everything should work (along with options for Git Bash to avoid any conflict between Git Bash with Cygwin).

If everything works better on Git Bash, let's use Git Bash by default and deprecate Cygwin support.

[hoatle]

related: #509