handle extract of self hardlink

[StefanKarpinski]

• git_tools: change tree_hash to include empty directories
• test that overwrites work for all before/after types
• handle extract of self hardlink (fix #127)

[codecov[bot]]

Codecov Report

Merging #130 (86e1f01) into master (ab7846e) will increase coverage by 0.40%. The diff coverage is 100.00%.

:exclamation: Current head 86e1f01 differs from pull request most recent head 96affcf. Consider uploading reports for the commit 96affcf to get more accurate results

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   97.28%   97.69%   +0.40%     
==========================================
  Files           4        4              
  Lines         774      736      -38     
==========================================
- Hits          753      719      -34     
+ Misses         21       17       -4     

Impacted Files	Coverage Δ
src/extract.jl	98.00% <100.00%> (-0.12%)	:arrow_down:
src/create.jl	100.00% <0.00%> (ø)
src/header.jl	94.44% <0.00%> (+1.11%)	:arrow_up:
src/Tar.jl	94.11% <0.00%> (+3.31%)	:arrow_up:

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