t1300: make tests more robust with non-default ref backends

The t1300 test suite exercises the git-config(1) tool. To do so, the
test overwrites ".git/config" to contain custom contents in several
places with code like the following:

```
cat > .git/config <<\EOF
...
EOF
```

While this is easy enough to do, it may create problems when using a
non-default repository format because this causes us to overwrite the
repository format version as well as any potential extensions. With the
upcoming "reftable" ref backend the result is that Git would try to
access refs via the "files" backend even though the repository has been
initialized with the "reftable" backend, which will cause failures when
trying to access any refs.

Ideally, we would rewrite the whole test suite to not depend on state
written by previous tests, but that would result in a lot of changes in
this test suite. Instead, we only refactor tests which access the refdb
to be more robust by using their own separate repositories, which allows
us to be more careful and not discard required extensions.

Note that we also have to touch up how the CUSTOM_CONFIG_FILE gets
accessed. This environment variable contains the relative path to a
custom config file which we're setting up. But because we are now using
subrepositories, this relative path will not be found anymore because
our working directory changes. This issue is addressed by storing the
absolute path to the file in CUSTOM_CONFIG_FILE instead.

Signed-off-by: Patrick Steinhardt <[email protected]>
Reviewed-by: Christian Couder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t1300-config.sh

@@ -1098,15 +1098,20 @@ test_expect_success SYMLINKS 'symlink to nonexistent configuration' '
 	test_must_fail git config --file=linktolinktonada --list
 '
 
-test_expect_success 'check split_cmdline return' "
-	git config alias.split-cmdline-fix 'echo \"' &&
-	test_must_fail git split-cmdline-fix &&
-	echo foo > foo &&
-	git add foo &&
-	git commit -m 'initial commit' &&
-	git config branch.main.mergeoptions 'echo \"' &&
-	test_must_fail git merge main
-"
+test_expect_success 'check split_cmdline return' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		git config alias.split-cmdline-fix "echo \"" &&
+		test_must_fail git split-cmdline-fix &&
+		echo foo >foo &&
+		git add foo &&
+		git commit -m "initial commit" &&
+		git config branch.main.mergeoptions "echo \"" &&
+		test_must_fail git merge main
+	)
+'
 
 test_expect_success 'git -c "key=value" support' '
 	cat >expect <<-\EOF &&
@@ -1157,10 +1162,16 @@ test_expect_success 'git -c works with aliases of builtins' '
 '
 
 test_expect_success 'aliases can be CamelCased' '
-	test_config alias.CamelCased "rev-parse HEAD" &&
-	git CamelCased >out &&
-	git rev-parse HEAD >expect &&
-	test_cmp expect out
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit A &&
+		git config alias.CamelCased "rev-parse HEAD" &&
+		git CamelCased >out &&
+		git rev-parse HEAD >expect &&
+		test_cmp expect out
+	)
 '
 
 test_expect_success 'git -c does not split values on equals' '
@@ -2009,11 +2020,11 @@ test_expect_success '--show-origin getting a single key' '
 '
 
 test_expect_success 'set up custom config file' '
-	CUSTOM_CONFIG_FILE="custom.conf" &&
-	cat >"$CUSTOM_CONFIG_FILE" <<-\EOF
+	cat >"custom.conf" <<-\EOF &&
 	[user]
 		custom = true
 	EOF
+	CUSTOM_CONFIG_FILE="$(test-tool path-utils real_path custom.conf)"
 '
 
 test_expect_success !MINGW 'set up custom config file with special name characters' '
@@ -2052,22 +2063,33 @@ test_expect_success '--show-origin stdin with file include' '
 '
 
 test_expect_success '--show-origin blob' '
-	blob=$(git hash-object -w "$CUSTOM_CONFIG_FILE") &&
-	cat >expect <<-EOF &&
-	blob:$blob	user.custom=true
-	EOF
-	git config --blob=$blob --show-origin --list >output &&
-	test_cmp expect output
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		blob=$(git hash-object -w "$CUSTOM_CONFIG_FILE") &&
+		cat >expect <<-EOF &&
+		blob:$blob	user.custom=true
+		EOF
+		git config --blob=$blob --show-origin --list >output &&
+		test_cmp expect output
+	)
 '
 
 test_expect_success '--show-origin blob ref' '
-	cat >expect <<-\EOF &&
-	blob:main:custom.conf	user.custom=true
-	EOF
-	git add "$CUSTOM_CONFIG_FILE" &&
-	git commit -m "new config file" &&
-	git config --blob=main:"$CUSTOM_CONFIG_FILE" --show-origin --list >output &&
-	test_cmp expect output
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		cat >expect <<-\EOF &&
+		blob:main:custom.conf	user.custom=true
+		EOF
+		cp "$CUSTOM_CONFIG_FILE" custom.conf &&
+		git add custom.conf &&
+		git commit -m "new config file" &&
+		git config --blob=main:custom.conf --show-origin --list >output &&
+		test_cmp expect output
+	)
 '
 
 test_expect_success '--show-origin with --default' '