commit test: test_set_editor in each test

t/t7507-commit-verbose.sh was using a global test_set_editor call to
build its environment.

Improve robustness against global state changes by having only tests
which intend to use the $EDITOR to check for presence of a diff in the
editor set up the test-editor to use check-for-diff rather than relying
upon the editor set once at script start.

Besides being in line with current practices, it also allows the tests
which set GIT_EDITOR=cat manually to avoid using a subshell and simplify
their logic.

Signed-off-by: Caleb Thompson <caleb@calebthompson.io>
commit: support commit.verbose and --no-verbose

Add a new configuration variable commit.verbose to implicitly pass
`--verbose` to `git-commit`. Add `--no-verbose` to commit to negate that
setting.

Signed-off-by: Caleb Thompson <caleb@calebthompson.io>