fix bugs handling unusual tarballs with directory after contents

[StefanKarpinski]

This fixes three related bugs when processing tarballs where a directory entry appears after some contents of that directory. This is unusual since neither command-line tar nor Tar.create will produce tarballs like this, and I only dicovered the bug when I accidentally modified Tar.create to emit directory entries after emiting everything inside of the directory. Each of extract, tree_hash and rewrite did this wrong in slightly different ways previously. Now they all (hopefully) do the right thing, and we test that they each handle a manually generated tarball with this kind of unusual ordering.

[codecov[bot]]

Codecov Report

Merging #105 (37c8e83) into master (008a3f2) will increase coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head 37c8e83 differs from pull request most recent head d0e2838. Consider uploading reports for the commit d0e2838 to get more accurate results

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   96.26%   96.29%   +0.02%     
==========================================
  Files           4        4              
  Lines         643      648       +5     
==========================================
+ Hits          619      624       +5     
  Misses         24       24              

Impacted Files	Coverage Δ
src/create.jl	97.07% <100.00%> (+0.01%)	:arrow_up:
src/extract.jl	96.37% <100.00%> (+0.03%)	:arrow_up:
src/Tar.jl	96.10% <0.00%> (+0.05%)	:arrow_up:

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