config tests: cover blind spots in git_die_config() tests

config tests: cover blind spots in git_die_config() tests

There were no tests checking for the output of the git_die_config()
function in the config API, added in 5a80e97c827 (config: add
`git_die_config()` to the config-set API, 2014-08-07). We only tested
"test_must_fail", but didn't assert the output.

We need tests for this because a subsequent commit will alter the
return value of git_config_get_value_multi(), which is used to get the
config values in the git_die_config() function. This test coverage
helps to build confidence in that subsequent change.

These tests cover different interactions with git_die_config():

- The "notes.mergeStrategy" test in
  "t/t3309-notes-merge-auto-resolve.sh" is a case where a function
  outside of config.c (git_config_get_notes_strategy()) calls
  git_die_config().

- The "gc.pruneExpire" test in "t5304-prune.sh" is a case where
  git_config_get_expiry() calls git_die_config(), covering a different
  "type" than the "string" test for "notes.mergeStrategy".

- The "fetch.negotiationAlgorithm" test in
  "t/t5552-skipping-fetch-negotiator.sh" is a case where
  git_config_get_string*() calls git_die_config().

We also cover both the "from command-line config" and "in file..at
line" cases here.

The clobbering of existing ".git/config" files here is so that we're
not implicitly testing the line count of the default config.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>