issue #274: new filemode package

[alcortesm]

Fix #274

[codecov]

Codecov Report

Merging #297 into master will increase coverage by 0.17%.
The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage   77.04%   77.22%   +0.17%     
==========================================
  Files         116      117       +1     
  Lines        7904     7948      +44     
==========================================
+ Hits         6090     6138      +48     
+ Misses       1162     1159       -3     
+ Partials      652      651       -1

Impacted Files	Coverage Δ
plumbing/object/file.go	76.08% <100%> (ø)	✅
plumbing/filemode/filemode.go	100% <100%> (ø)
plumbing/object/treenoder.go	79.62% <100%> (-1.41%)	❌
plumbing/object/difftree.go	90.9% <100%> (-0.4%)	❌
worktree.go	56.69% <56.25%> (ø)	✅
plumbing/object/tree.go	74.37% <83.33%> (+0.19%)	✅

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