Merge branch 'rr/test-make-sure-we-have-git'

Only the first test t0000 in the test suite made sure we have built
Git to be tested; move the check to test-lib so that it applies to
all tests equally.

* rr/test-make-sure-we-have-git:
t/test-lib: make sure Git has already been built

diff --cc t/t0000-basic.sh
Simple merge

diff --cc t/test-lib.sh
Simple merge