clone: fix colliding file detection on APFS

clone: fix colliding file detection on APFS

Commit b878579ae7 (clone: report duplicate entries on case-insensitive
filesystems - 2018-08-17) adds a warning to user when cloning a repo
with case-sensitive file names on a case-insensitive file system. The
"find duplicate file" check was doing by comparing inode number (and
only fall back to fspathcmp() when inode is known to be unreliable
because fspathcmp() can't cover all case folding cases).

The inode check is very simple, and wrong. It compares between a
32-bit number (sd_ino) and potentially a 64-bit number (st_ino). When
an inode is larger than 2^32 (which seems to be the case for APFS), it
will be truncated and stored in sd_ino, but comparing with itself will
fail.

As a result, instead of showing a pair of files that have the same
name, we show just one file (marked before the beginning of the
loop). We fail to find the original one.

The fix could be just a simple type cast (*)

    dup->ce_stat_data.sd_ino == (unsigned int)st->st_ino

but this is no longer a reliable test, there are 4G possible inodes
that can match sd_ino because we only match the lower 32 bits instead
of full 64 bits.

There are two options to go. Either we ignore inode and go with
fspathcmp() on Apple platform. This means we can't do accurate inode
check on HFS anymore, or even on APFS when inode numbers are still
below 2^32.

Or we just to to reduce the odds of matching a wrong file by checking
more attributes, counting mostly on st_size because st_xtime is likely
the same. This patch goes with this direction, hoping that false
positive chances are too small to be seen in practice.

While at there, enable the test on Cygwin (verified working by Ramsay
Jones)

(*) this is also already done inside match_stat_data()

Reported-by: Carlo Arenas <carenas@gmail.com>
Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>