Improving / Exposing Git-Trees related features for Git-Trees API.

[Kasi-R]

Additions to allow/improve Git-Trees API.

• Exposes TreeEntry.mode through TreeEntry.Mode()
• Fixes EntryMode consts (changed to hex)
• Added revision parsing for trees (HEAD, HEAD~1, etc).

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@578ad8f). Click here to learn what that means.
The diff coverage is 0%.

@@           Coverage Diff            @@
##             master    #123   +/-   ##
========================================
  Coverage          ?   32.9%           
========================================
  Files             ?      28           
  Lines             ?    1781           
  Branches          ?       0           
========================================
  Hits              ?     586           
  Misses            ?    1118           
  Partials          ?      77

Impacted Files	Coverage Δ
tree_entry.go	67.67% <0%> (ø)
repo_tree.go	22.72% <0%> (ø)
tree.go	53.06% <0%> (ø)

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[filipnavara]

The implementation of ListEntries/ListEntriesRecursive is broken. They both cache the output in the same variable. Whichever is called first returns the correct value, but calling the other one will result in incorrect result.