Remove `LibGit2` dependency #206

Job	Run time
Documentation	56s
Julia 1.0 - ubuntu-latest - x64	1m 22s
Julia 1.0 - windows-latest - x64	3m 0s
Julia 1.0 - macos-latest - x64	1m 59s
Julia 1 - windows-latest - x64	1m 17s
Julia 1 - ubuntu-latest - x64	33s
Julia 1 - macos-latest - x64	1m 17s
Julia nightly - macos-latest - x64	1m 18s
Julia nightly - windows-latest - x64	1m 31s
Julia nightly - ubuntu-latest - x64	51s
	14m 4s

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