It's really important for `git-check-assertions` blocks to be 'obviously
correct', so that we don't need to... test the tests that test the tests.
Let's introduce a little DSL that is less error-prone than a plain bash
script.
```git-check-assertions
run ./test/git-check-assertions.bats
assert_success
git checkout HEAD~ bin
run ./test/git-check-assertions.bats
assert_failure
assert_output --partial "not ok 10 should restore the commit between assertion blocks in one commit message"
```
```git-check-assertions
run test/git-check-assertions.bats
assert_success
git checkout HEAD~1 bin
run test/git-check-assertions.bats
assert_failure
assert_output --partial "file exists, but it was expected to be absent"
```
```git-check-assertions
run test/git-check-assertions.bats
assert_success
git checkout HEAD~1 bin
run test/git-check-assertions.bats
assert_failure
```
```git-check-assertions
run test/git-check-assertions.bats
assert_success
git checkout HEAD~1 bin
run test/git-check-assertions.bats
assert_failure
assert_output --partial "not ok 2 should refuse to run when the index has changes"
assert_output --partial "not ok 3 should refuse to run when the working tree has changes"
assert_output --partial "not ok 4 should refuse to run when there are untracked files"
```
```git-check-assertions
run ./test/git-check-assertions.bats
assert_failure
assert_output --partial "not ok 9 should restore index when finished"
assert_output --partial "not ok 10 should restore index between commits"
assert_output --partial "not ok 11 should restore index after a failure"
assert_output --partial "not ok 13 you can use git checkout in an assertion to assert against a mixed version of the code"
```
This test currently doesn't pass:
```git-check-assertions
run ./test/git-check-assertions.bats
assert_failure
assert_output --partial "file is greater than 0 byte"
```
```git-check-assertions
sed -i 's/HEAD~/HEAD/' test/git-check-assertions.bats
run test/git-check-assertions.bats
assert_failure
assert_output --partial "Expected command to fail, but it succeeded."
```
