Merge branch 'fc/completion-test-simplification'

Clean up completion tests.  Use of conslidated helper may make
instrumenting one particular test during debugging of the test
itself, but I think that issue should be addressed in some other
way (e.g. making sure individual tests in 9902 can be skipped).

* fc/completion-test-simplification:
  completion: simplify __gitcomp() test helper
  completion: refactor __gitcomp related tests
  completion: consolidate test_completion*() tests
  completion: simplify tests using test_completion_long()
  completion: standardize final space marker in tests
  completion: add comment for test_completion()

diff --cc t/t9902-completion.sh
Simple merge