t/20-usage: Also match newer Git error message
t/20-usage: Also match newer Git error message

Somewhere between Git 2.11 and 2.17 they changed the 'Not a git
repository...' message not to have an initial capital.

cjs-cynic-net/git-commit-filetree,cjs-cynic-net/git-commit-filetree,cjs-cynic-net/git-commit-filetree