feat: set env var with git params when available

[calebcartwright]

Changes

• Make git params available as an environment variable for hook scripts via $RUSTY_HOOK_GIT_PARAMS
• Add unit tests for handling of presence/absence of git params

Related Issues

• Closes Make git params available as environment variable #25

[codecov]

Codecov Report

Merging #123 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #123   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          139       143    +4     
=========================================
+ Hits           139       143    +4     

Impacted Files	Coverage Δ
src/rusty_hook.rs	100.00% <100.00%> (ø)

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