mingw: factor out the retry logic

mingw: factor out the retry logic

In several places, Git's Windows-specific code follows the pattern where
it tries to perform an operation, and retries several times when that
operation fails, sleeping an increasing amount of time, before finally
giving up and asking the user whether to rety (after, say, closing an
editor that held a handle to a file, preventing the operation from
succeeding).

This logic is a bit hard to use, and inconsistent:
`mingw_unlink()` and `mingw_rmdir()` duplicate the code to retry,
and both of them do so incompletely. They also do not restore `errno` if the
user answers 'no'.

Introduce a `retry_ask_yes_no()` helper function that handles retry with
small delay, asking the user, and restoring `errno`.

Note that in `mingw_unlink()`, we include the `_wchmod()` call in the
retry loop (which may fail if the file is locked exclusively).

In `mingw_rmdir()`, we include special error handling in the retry loop.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>