t: prepare `test_must_fail ()` for `set -e`

t: prepare `test_must_fail ()` for `set -e`

The helper function `test_must_fail ()` executes a specific Git command
that may or may not fail in a specific way. This is done by executing
the command in question and then comparing its exit code against a set
of conditions.

This works, but once we run our test suite with `set -e` we may bail out
of `test_must_fail ()` early in case the command actually fails, even
though we expect it to fail. Prepare for this change by handling the
failed case with `||`.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>