Windows: Fix intermittent failures of t7701

Windows: Fix intermittent failures of t7701

The last test case checks whether unpacked objects receive the time stamp
of the pack file. Due to different implementations of stat(2) by MSYS and
our version in compat/mingw.c, the test fails in about half of the test
runs.

Note the following facts:

- The test uses perl's -M operator to compare the time stamps. Since we
  depend on MSYS perl, the result of this operator is based on MSYS's
  implementation of the stat(2) call.

- NTFS on Windows records fractional seconds.

- The MSYS implementation of stat(2) *rounds* fractional seconds to full
  seconds instead of truncating them. This becomes obvious by comparing the
  modification times reported by 'ls --full-time $f' and 'stat $f' for
  various files $f.

- Our implementation of stat(2) in compat/mingw.c *truncates* to full
  seconds.

The consequence of this is that

- add_packed_git() picks up a truncated whole second modification time
  from the pack file time stamp, which is then used for the loose objects,
  while the pack file retains its time stamp in fractional seconds;

- but the test case compares the pack file's rounded modification times
  to the loose objects' truncated modification times.

And half of the time the rounded modification time is not the same as its
truncated modification time.

The fix is that we replace perl by 'test-chmtime -v +0', which prints the
truncated whole-second mtime without modifying it.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>