Re: [PATCH] t5310-pack-bitmaps: fix bogus 'pack-objects to file can use bitmap' test

[Matthew DeVore <matvore@google.com>]

I would encourage use of an existing function to check for emptiness,
but require a particular argument for it to be considered "the right
way:"

test_cmp /dev/null actual

This means less vocabulary to memorize for test writers. It's usually a
code smell to have special logic for a specific value for a specific
argument - a sign that a separate function ought to be created - but
since we want to add an error or warning in test_cmp anyway when
<EXPECTED> is empty, I think this special logic is OK.

As for comparing against a file that *might* be empty, like a utility
function, might I suggest requiring the file name be formatted in a
specific way if it may be empty? Like require a certain substring. Then
the syntax for comparison would be:

# If the emptiness is unconditional
test_cmp /dev/null actual

# If the emptiness is unknown ahead of time
test_cmp maybe_empty_expected actual

Then, issue an error for something like:
> expected && test_cmp expected actual

which says: "
Use test_cmp /dev/null <ACTUAL> to verify a file is empty.
If the <EXPECTED> file may or may not be empty (as in a utility
function), include the string "maybe_empty" in the <EXPECTED> file name.
"