t0008: don't rely on default ".git/info/exclude"

Change a test added in 368aa52952 (add git-check-ignore sub-command,
2013-01-06) to clobber .git/info/exclude rather than append to
it.

These tests would break if the "templates/info--exclude" file added in
d3af621b14 (Redo the templates generation and installation.,
2005-08-06) wasn't exactly 6 lines (of only comments).

Let's instead clobber the default .git/info/excludes file, and test
only our own expected content. This is not strictly needed for
anything in this series, but is a good cleanup while we're at it.

As discussed in the preceding commit a lot of things depend on the
"info" directory being created, but this was the only test that relied
on the specific content in the "templates/info--exclude" file.

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

t/t0008-ignores.sh

@@ -225,7 +225,7 @@ test_expect_success 'setup' '
 		!globaltwo
 		globalthree
 	EOF
-	cat <<-\EOF >>.git/info/exclude
+	cat <<-\EOF >.git/info/exclude
 		per-repo
 	EOF
 '
@@ -543,9 +543,9 @@ test_expect_success_multi 'submodule from subdirectory' '' '
 
 test_expect_success 'global ignore not yet enabled' '
 	expect_from_stdin <<-\EOF &&
-		.git/info/exclude:7:per-repo	per-repo
+		.git/info/exclude:1:per-repo	per-repo
 		a/.gitignore:2:*three	a/globalthree
-		.git/info/exclude:7:per-repo	a/per-repo
+		.git/info/exclude:1:per-repo	a/per-repo
 	EOF
 	test_check_ignore "-v globalone per-repo a/globalthree a/per-repo not-ignored a/globaltwo"
 '
@@ -566,10 +566,10 @@ test_expect_success 'global ignore with -v' '
 	enable_global_excludes &&
 	expect_from_stdin <<-EOF &&
 		$global_excludes:1:globalone	globalone
-		.git/info/exclude:7:per-repo	per-repo
+		.git/info/exclude:1:per-repo	per-repo
 		$global_excludes:3:globalthree	globalthree
 		a/.gitignore:2:*three	a/globalthree
-		.git/info/exclude:7:per-repo	a/per-repo
+		.git/info/exclude:1:per-repo	a/per-repo
 		$global_excludes:2:!globaltwo	globaltwo
 	EOF
 	test_check_ignore "-v globalone per-repo globalthree a/globalthree a/per-repo not-ignored globaltwo"