t: prefer "git config --file" to GIT_CONFIG

Doing:

  GIT_CONFIG=foo git config ...

is equivalent to:

  git config --file=foo ...

The latter is easier to read and slightly less error-prone,
because of issues with one-shot variables and shell
functions (e.g., you cannot use the former with
test_must_fail).

Note that we explicitly leave one case in t1300 which checks
the same operation on both GIT_CONFIG and "git config
--file". They are equivalent in the code these days, but
this will make sure it remains so.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1300-repo-config.sh

@@ -461,7 +461,7 @@ test_expect_success 'new variable inserts into proper section' '
 	test_cmp expect .git/config
 '
 
-test_expect_success 'alternative GIT_CONFIG (non-existing file should fail)' '
+test_expect_success 'alternative --file (non-existing file should fail)' '
 	test_must_fail git config --file non-existing-config -l
 '
 
@@ -495,10 +495,10 @@ test_expect_success 'refer config from subdirectory' '
 
 '
 
-test_expect_success 'refer config from subdirectory via GIT_CONFIG' '
+test_expect_success 'refer config from subdirectory via --file' '
 	(
 		cd x &&
-		GIT_CONFIG=../other-config git config --get ein.bahn >actual &&
+		git config --file=../other-config --get ein.bahn >actual &&
 		test_cmp expect actual
 	)
 '
@@ -510,8 +510,8 @@ cat > expect << EOF
 	park = ausweis
 EOF
 
-test_expect_success '--set in alternative GIT_CONFIG' '
+test_expect_success '--set in alternative file' '
-	GIT_CONFIG=other-config git config anwohner.park ausweis &&
+	git config --file=other-config anwohner.park ausweis &&
 	test_cmp expect other-config
 '
 
@@ -942,11 +942,11 @@ test_expect_success 'inner whitespace kept verbatim' '
 
 test_expect_success SYMLINKS 'symlinked configuration' '
 	ln -s notyet myconfig &&
-	GIT_CONFIG=myconfig git config test.frotz nitfol &&
+	git config --file=myconfig test.frotz nitfol &&
 	test -h myconfig &&
 	test -f notyet &&
-	test "z$(GIT_CONFIG=notyet git config test.frotz)" = znitfol &&
+	test "z$(git config --file=notyet test.frotz)" = znitfol &&
-	GIT_CONFIG=myconfig git config test.xyzzy rezrov &&
+	git config --file=myconfig test.xyzzy rezrov &&
 	test -h myconfig &&
 	test -f notyet &&
 	cat >expect <<-\EOF &&
@@ -954,8 +954,8 @@ test_expect_success SYMLINKS 'symlinked configuration' '
 	rezrov
 	EOF
 	{
-		GIT_CONFIG=notyet git config test.frotz &&
+		git config --file=notyet test.frotz &&
-		GIT_CONFIG=notyet git config test.xyzzy
+		git config --file=notyet test.xyzzy
 	} >actual &&
 	test_cmp expect actual
 '

t/t1302-repo-version.sh

@@ -19,7 +19,7 @@ test_expect_success 'setup' '
 
 	test_create_repo "test" &&
 	test_create_repo "test2" &&
-	GIT_CONFIG=test2/.git/config git config core.repositoryformatversion 99
+	git config --file=test2/.git/config core.repositoryformatversion 99
 '
 
 test_expect_success 'gitdir selection on normal repos' '

t/t7400-submodule-basic.sh

@@ -249,8 +249,7 @@ test_expect_success 'submodule add in subdirectory with relative path should fai
 '
 
 test_expect_success 'setup - add an example entry to .gitmodules' '
-	GIT_CONFIG=.gitmodules \
+	git config --file=.gitmodules submodule.example.url git://example.com/init.git
-	git config submodule.example.url git://example.com/init.git
 '
 
 test_expect_success 'status should fail for unmapped paths' '
@@ -264,7 +263,7 @@ test_expect_success 'setup - map path in .gitmodules' '
 	path = init
 EOF
 
-	GIT_CONFIG=.gitmodules git config submodule.example.path init &&
+	git config --file=.gitmodules submodule.example.path init &&
 
 	test_cmp expect .gitmodules
 '

t/t9130-git-svn-authors-file.sh

@@ -67,7 +67,7 @@ test_expect_success 'fetch fails on ee' '
 	'
 
 tmp_config_get () {
-	GIT_CONFIG=.git/svn/.metadata git config --get "$1"
+	git config --file=.git/svn/.metadata --get "$1"
 }
 
 test_expect_success 'failure happened without negative side effects' '

t/t9154-git-svn-fancy-glob.sh

@@ -22,7 +22,7 @@ test_expect_success 'add red branch' "
 	"
 
 test_expect_success 'add gre branch' "
-	GIT_CONFIG=.git/svn/.metadata git config --unset svn-remote.svn.branches-maxRev &&
+	git config --file=.git/svn/.metadata --unset svn-remote.svn.branches-maxRev &&
 	git config svn-remote.svn.branches 'branches/{red,gre}:refs/remotes/*' &&
 	git svn fetch &&
 	git rev-parse refs/remotes/red &&
@@ -31,7 +31,7 @@ test_expect_success 'add gre branch' "
 	"
 
 test_expect_success 'add green branch' "
-	GIT_CONFIG=.git/svn/.metadata git config --unset svn-remote.svn.branches-maxRev &&
+	git config --file=.git/svn/.metadata --unset svn-remote.svn.branches-maxRev &&
 	git config svn-remote.svn.branches 'branches/{red,green}:refs/remotes/*' &&
 	git svn fetch &&
 	git rev-parse refs/remotes/red &&
@@ -40,7 +40,7 @@ test_expect_success 'add green branch' "
 	"
 
 test_expect_success 'add all branches' "
-	GIT_CONFIG=.git/svn/.metadata git config --unset svn-remote.svn.branches-maxRev &&
+	git config --file=.git/svn/.metadata --unset svn-remote.svn.branches-maxRev &&
 	git config svn-remote.svn.branches 'branches/*:refs/remotes/*' &&
 	git svn fetch &&
 	git rev-parse refs/remotes/red &&