In c4849b2dab87, the "commit" function was extracted into a testutil for re-use.
However, test-encoding.t was skipped over in that changeset, as I was seeing
unexplained test failures.  Since those test failures have now been explained
(and fixed), this changeset performs the same extraction on test-encoding.t as
was done on all the other tests.

The version of fn_git_commit that was used in testutil redirected all output
(including errors) to /dev/null, which didn't match the expectations of this
test.  The test utility functions for commit/tag now no longer throw away error
output, instead leaving it to individual tests to decide if error output should
be ignored.