Print libgit2 version during cmd check

tests/libgit2.R

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+library(gert)

tests/libgit2.Rout.save

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+
+R version 3.5.3 (2019-03-11) -- "Great Truth"
+Copyright (C) 2019 The R Foundation for Statistical Computing
+Platform: x86_64-apple-darwin15.6.0 (64-bit)
+
+R is free software and comes with ABSOLUTELY NO WARRANTY.
+You are welcome to redistribute it under certain conditions.
+Type 'license()' or 'licence()' for distribution details.
+
+R is a collaborative project with many contributors.
+Type 'contributors()' for more information and
+'citation()' on how to cite R or R packages in publications.
+
+Type 'demo()' for some demos, 'help()' for on-line help, or
+'help.start()' for an HTML browser interface to help.
+Type 'q()' to quit R.
+
+> library(gert)
+>