When I launch *any* version of Python using py.exe from Git Bash, the up and down arrows clear the line buffer instead of navigating history.

The following things all work as intended:

* py.exe in cmd.exe
* py.exe in Git Bash using winpty
* Python 2.7, 3.5, 3.6 when run directly

The only thing that appears to trigger this bug is using the launcher in Git Bash without winpty.

Tested with the following versions of Git Bash:

* GNU bash, version 4.3.42(2)-release (x86_64-pc-msys)
* GNU bash, version 4.3.46(2)-release (x86_64-pc-msys)

Unfortunately I can't point to exactly which version of the launcher introduced this bug. Initially I was certain this only happened when I upgraded from 3.5.2 to 3.6.0, but now I've checked another machine with 3.5.2 and 3.4.4, and the bug appears there too.