Fix error detection for "ls-tree" output

Summary: Fixes T4223. The output of `ls-tree` is partially delimited by spaces
and partially delimited by `\t`. The code I added in D7744 to help debug the
issue in T4159 doesn't work properly for files with 7 or more bytes in their
filesize, because the internals use `%7s`.

Auditors: btrahan