Summary:
https://github.com/facebook/rocksdb/issues/8031 broke internal tests. This should fix but also preserve
the intended capability of getting git commit id when hg not used
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9047
Test Plan: already broken ¯\\_(ツ)_/¯
Reviewed By: zhichao-cao
Differential Revision: D31732198
Pulled By: pdillinger
fbshipit-source-id: 7dba8531ddca55a6de5e04978a1a1601aae4cee9