t: remove \{m,n\} from BRE grep usage

The CodingGuidelines says we should avoid \{m,n\} in BRE usage.
And their usages in our code base is limited, and subjectively
hard to read.

Replace them with ERE.

Except for "0\{40\}" which would be changed to "$ZERO_OID",
which is a better value for testing with:
GIT_TEST_DEFAULT_HASH=sha256

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -201,8 +201,8 @@ test_expect_success 'git branch -M baz bam should succeed when baz is checked ou
 
 test_expect_success 'git branch -M baz bam should add entries to .git/logs/HEAD' '
 	msg="Branch: renamed refs/heads/baz to refs/heads/bam" &&
-	grep " 0\{40\}.*$msg$" .git/logs/HEAD &&
-	grep "^0\{40\}.*$msg$" .git/logs/HEAD
+	grep " $ZERO_OID.*$msg$" .git/logs/HEAD &&
+	grep "^$ZERO_OID.*$msg$" .git/logs/HEAD
 '
 
 test_expect_success 'git branch -M should leave orphaned HEAD alone' '

t/t3305-notes-fanout.sh

@@ -9,7 +9,7 @@ path_has_fanout() {
 	path=$1 &&
 	fanout=$2 &&
 	after_last_slash=$(($(test_oid hexsz) - $fanout * 2)) &&
-	echo $path | grep -q "^\([0-9a-f]\{2\}/\)\{$fanout\}[0-9a-f]\{$after_last_slash\}$"
+	echo $path | grep -q -E "^([0-9a-f]{2}/){$fanout}[0-9a-f]{$after_last_slash}$"
 }
 
 touched_one_note_with_fanout() {

t/t3404-rebase-interactive.sh

@@ -1244,9 +1244,9 @@ test_expect_success 'short commit ID collide' '
 		test $colliding_id = "$(git rev-parse HEAD | cut -c 1-4)" &&
 		grep "^pick $colliding_id " \
 			.git/rebase-merge/git-rebase-todo.tmp &&
-		grep "^pick [0-9a-f]\{$hexsz\}" \
+		grep -E "^pick [0-9a-f]{$hexsz}" \
 			.git/rebase-merge/git-rebase-todo &&
-		grep "^pick [0-9a-f]\{$hexsz\}" \
+		grep -E "^pick [0-9a-f]{$hexsz}" \
 			.git/rebase-merge/git-rebase-todo.backup &&
 		git rebase --continue
 	) &&
@@ -1261,7 +1261,7 @@ test_expect_success 'respect core.abbrev' '
 		set_cat_todo_editor &&
 		test_must_fail git rebase -i HEAD~4 >todo-list
 	) &&
-	test 4 = $(grep -c "pick [0-9a-f]\{12,\}" todo-list)
+	test 4 = $(grep -c -E "pick [0-9a-f]{12,}" todo-list)
 '
 
 test_expect_success 'todo count' '

t/t5550-http-fetch-dumb.sh

@@ -234,7 +234,7 @@ test_expect_success 'http-fetch --packfile' '
 		--index-pack-arg=--keep \
 		"$HTTPD_URL"/dumb/repo_pack.git/$p >out &&
 
-	grep "^keep.[0-9a-f]\{16,\}$" out &&
+	grep -E "^keep.[0-9a-f]{16,}$" out &&
 	cut -c6- out >packhash &&
 
 	# Ensure that the expected files are generated

t/t5702-protocol-v2.sh

@@ -1001,7 +1001,7 @@ test_expect_success 'part of packfile response provided as URI' '
 	do
 		git verify-pack --object-format=$(test_oid algo) --verbose $idx >out &&
 		{
-			grep "^[0-9a-f]\{16,\} " out || :
+			grep -E "^[0-9a-f]{16,} " out || :
 		} >out.objectlist &&
 		if test_line_count = 1 out.objectlist
 		then