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 5a80e97c82 (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>

t/t3309-notes-merge-auto-resolve.sh

@@ -360,7 +360,12 @@ test_expect_success 'merge z into y with invalid strategy => Fail/No changes' '
 
 test_expect_success 'merge z into y with invalid configuration option => Fail/No changes' '
 	git config core.notesRef refs/notes/y &&
-	test_must_fail git -c notes.mergeStrategy="foo" notes merge z &&
+	cat >expect <<-\EOF &&
+	error: unknown notes merge strategy foo
+	fatal: unable to parse '\''notes.mergeStrategy'\'' from command-line config
+	EOF
+	test_must_fail git -c notes.mergeStrategy="foo" notes merge z 2>actual &&
+	test_cmp expect actual &&
 	# Verify no changes (y)
 	verify_notes y y
 '

t/t5304-prune.sh

@@ -64,8 +64,16 @@ test_expect_success 'gc: implicit prune --expire' '
 '
 
 test_expect_success 'gc: refuse to start with invalid gc.pruneExpire' '
-	git config gc.pruneExpire invalid &&
-	test_must_fail git gc
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	>repo/.git/config &&
+	git -C repo config gc.pruneExpire invalid &&
+	cat >expect <<-\EOF &&
+	error: Invalid gc.pruneexpire: '\''invalid'\''
+	fatal: bad config variable '\''gc.pruneexpire'\'' in file '\''.git/config'\'' at line 2
+	EOF
+	test_must_fail git -C repo gc 2>actual &&
+	test_cmp expect actual
 '
 
 test_expect_success 'gc: start with ok gc.pruneExpire' '

t/t5552-skipping-fetch-negotiator.sh

@@ -3,6 +3,22 @@
 test_description='test skipping fetch negotiator'
 . ./test-lib.sh
 
+test_expect_success 'fetch.negotiationalgorithm config' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	cat >repo/.git/config <<-\EOF &&
+	[fetch]
+	negotiationAlgorithm
+	EOF
+	cat >expect <<-\EOF &&
+	error: missing value for '\''fetch.negotiationalgorithm'\''
+	fatal: bad config variable '\''fetch.negotiationalgorithm'\'' in file '\''.git/config'\'' at line 2
+	EOF
+	test_expect_code 128 git -C repo fetch >out 2>actual &&
+	test_must_be_empty out &&
+	test_cmp expect actual
+'
+
 have_sent () {
 	while test "$#" -ne 0
 	do