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This seems like a very useful tool, and it provides useful information on our repository.

But in RHEL7 it gives this error: error: couldn't open Git repository: git rev-parse failed: Unknown option: -C

I assume it requires a later version of git.

git-sizer uses the -C option, which was added in git v1.8.5:

RHEL7 ships with git v1.8.3.1.

There might be other compatiblity issues, e.g.: https://github.com/github/git-sizer/issues/18

If you need modern (but stable and maintained by Rackspace) packages in CentOS check out https://ius.io/