feat: support loading commit metadata

[alexmt]

Closes #43
Closes #30

PR adds function repo.GetCommitMetadata to templates/triggers that returns commit metadata.

• implementation
• documentation
• tests

[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 1.46%.
The diff coverage is 26.38%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   53.98%   52.52%   -1.47%     
==========================================
  Files          25       25              
  Lines        1343     1388      +45     
==========================================
+ Hits          725      729       +4     
- Misses        513      551      +38     
- Partials      105      108       +3     

Impacted Files	Coverage Δ
cmd/tools/tools.go	33.33% <0.00%> (-3.63%)	⬇️
shared/settings/settings.go	58.90% <0.00%> (ø)
cmd/tools/context.go	15.00% <5.55%> (-4.05%)	⬇️
cmd/tools/template.go	61.60% <16.66%> (-1.13%)	⬇️
cmd/controller.go	31.45% <22.22%> (-1.60%)	⬇️
triggers/expr/repo/repo.go	41.17% <25.00%> (-11.46%)	⬇️
triggers/triggers.go	66.10% <90.00%> (ø)
triggers/expr/expr.go	100.00% <100.00%> (ø)

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