Update Mac juliarc.jl (fixes #6880)

git (bundled in ROOT/libexec/git-core) isn't included on the PATH used by Julia. This causes issues on OSX when using the package manager because GIT_EXEC_PATH does point to the bundled git tools. By adding git-core explicitly, all issues are fixed. The bundled git is used throughout.
Merge pull request #6992 from apleonhardt/apl/macgit2

Update Mac juliarc.jl (fixes #6880)