"Alexandr Miloslavskiy via GitGitGadget" [off-list ref]
writes:

From: Alexandr Miloslavskiy <redacted>

In my previous patches, `parse_pathspec_file()` was tested indirectly by
invoking `git reset` and `git commit` with properly crafted inputs. This
has some disadvantages:
1) A number of tests are copy&pasted for every command where
   `--pathspec-from-file` is supported. With just two commands, it
   wasn't too bad, but I'm going to extend support to many more
   commands, which would make a handful of low-value tests.
2) Tests are located in suboptimal test packages
3) Tests are indirect

That cuts both ways.  For a developer who is too narrowly focused
(because s/he spent enough time staring at the code), testing the
underlying machinery in a more direct way does feel attractive, but
at the same time, what matters to the end users is how well the
feature, when integrated into the commands they use (not the test
scaffolding like the "test-parse-pathspec-file" command), works.

So "indirect" is not necessarily a bad thing.