Windows: Work around intermittent failures in mingw_rename

We have replaced rename() with a version that can rename a file to a
destination that already exists. Nevertheless, many users, the author
included, observe failures in the code that are not reproducible.

The theory is that the failures are due to some other process that happens
to have opened the destination file briefly at the wrong moment. (And there
is no way on Windows to delete or replace a file that is currently open.)
The most likely candidate for such a process is a virus scanner. The
failure is more often observed while there is heavy git activity (for
example while the test suite is running or during a rebase operation).

We work around the failure by retrying the rename operation if it failed
due to ERROR_ACCESS_DENIED. The retries are delayed a bit: The first only
by giving up the time slice, the next after the minimal scheduling
granularity, and if more retries are needed, then we wait some non-trivial
amount of time with exponential back-off.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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