Add stream helper library

This library provides thread-unsafe fgets()- and fread()-like
functions where the caller does not have to supply a buffer.  It
maintains a couple of static buffers and provides an API to use
them.

[rr: allow input from files other than stdin]
[jn: with tests, documentation, and error handling improvements]

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0080-vcs-svn.sh

@@ -76,6 +76,60 @@ test_expect_success 'obj pool: high-water mark' '
 	test_cmp expected actual
 '
 
+test_expect_success 'line buffer' '
+	echo HELLO >expected1 &&
+	printf "%s\n" "" HELLO >expected2 &&
+	echo >expected3 &&
+	printf "%s\n" "" Q | q_to_nul >expected4 &&
+	printf "%s\n" foo "" >expected5 &&
+	printf "%s\n" "" foo >expected6 &&
+
+	test-line-buffer <<-\EOF >actual1 &&
+	5
+	HELLO
+	EOF
+
+	test-line-buffer <<-\EOF >actual2 &&
+	0
+
+	5
+	HELLO
+	EOF
+
+	q_to_nul <<-\EOF |
+	1
+	Q
+	EOF
+	test-line-buffer >actual3 &&
+
+	q_to_nul <<-\EOF |
+	0
+
+	1
+	Q
+	EOF
+	test-line-buffer >actual4 &&
+
+	test-line-buffer <<-\EOF >actual5 &&
+	5
+	foo
+	EOF
+
+	test-line-buffer <<-\EOF >actual6 &&
+	0
+
+	5
+	foo
+	EOF
+
+	test_cmp expected1 actual1 &&
+	test_cmp expected2 actual2 &&
+	test_cmp expected3 actual3 &&
+	test_cmp expected4 actual4 &&
+	test_cmp expected5 actual5 &&
+	test_cmp expected6 actual6
+'
+
 test_expect_success 'string pool' '
 	echo a does not equal b >expected.differ &&
 	echo a equals a >expected.match &&