Merge branch 'ib/test-selectively-run'

Allow specifying only certain individual test pieces to be run
using a range notation (e.g. "t1234-test.sh --run='1-4 6 8 9-'").

* ib/test-selectively-run:
t0000-*.sh: fix the GIT_SKIP_TESTS sub-tests
test-lib: '--run' to run only specific tests
test-lib: tests skipped by GIT_SKIP_TESTS say so
test-lib: document short options in t/README

diff --cc t/README
Simple merge

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