mingw: make failures to unlink or move raise a question

mingw: make failures to unlink or move raise a question

On Windows in case a program is accessing a file unlink or
move operations may fail. To give the user a chance to correct
this we simply wait until the user asks us to retry or fail.

This is useful because of the following use case which seem
to happen rarely but when it does it is a mess:

After making some changes the user realizes that he was on the
incorrect branch. When trying to change the branch some file
is still in use by some other process and git stops in the
middle of changing branches. Now the user has lots of files
with changes mixed with his own. This is especially confusing
on repositories that contain lots of files.

Although the recent implementation of automatic retry makes
this scenario much more unlikely lets provide a fallback as
a last resort.

Thanks to Albert Dvornik for disabling the question if users can't see it.

If the stdout of the command is connected to a terminal but the stderr
has been redirected, the odds are good that the user can't see any
question we print out to stderr.  This will result in a "mysterious
hang" while the app is waiting for user input.

It seems better to be conservative, and avoid asking for input
whenever the stderr is not a terminal, just like we do for stdin.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Albert Dvornik <dvornik+git@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>