fsck: report non-consecutive duplicate names in trees

fsck: report non-consecutive duplicate names in trees

Tree entries are sorted in path order, meaning that directory names get
a slash ('/') appended implicitly.  Git fsck checks if trees contains
consecutive duplicates, but due to that ordering there can be
non-consecutive duplicates as well if one of them is a directory and the
other one isn't.  Such a tree cannot be fully checked out.

Find these duplicates by recording candidate file names on a stack and
check candidate directory names against that stack to find matches.

Suggested-by: Brandon Williams <redacted>
Original-test-by: Brandon Williams <redacted>
Signed-off-by: René Scharfe <redacted>
Reviewed-by: Luke Diamand <redacted>
Signed-off-by: Junio C Hamano <redacted>