Use dev_t for device id (st_dev) from stat in setup_git_directory_gently()

Use dev_t for device id (st_dev) from stat in setup_git_directory_gently()

The original declaration was int, which seems to cause trouble on my
machine.  It causes spurious "filesystem boundary" errors when running
the testsuite.  The cause seems to be

  $ stat -c%d .
  2147549952

which is too large for a 32-bit int type.

Using the correct type, dev_t, solves the issue.  (Because I'm
paranoid and forgetful, I checked -- yes, Unix v7 had dev_t.)

Other uses of st_dev seem to be reasonably safe.   fill_stat_cache_info
truncates it to an 'unsigned int', but that value seems to be used only
to validate the cache, and only if USE_STDEV is defined.

Signed-off-by: Raja R Harinath <harinath@hurrynot.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>