mingw (git_terminal_prompt): work around BusyBox & WSL issues

When trying to query the user directly via /dev/tty, both WSL's bash and
BusyBox' bash emulation seem to have problems printing the value that
they just read. The bash just stops in those instances, does not even
execute any commands after the echo command.

Let's just work around this by running the Bash snippet only in MSYS2's
Bash: its `SHELL` variable has the `.exe` suffix, and neither WSL's nor
BusyBox' bash set the `SHELL` variable to a path with that suffix. In
the latter case, we simply exit with code 127 (indicating that the
command was not found) and fall back to the CONIN$/CONOUT$ method
quietly.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho