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>

.gitignore

@@ -166,6 +166,7 @@
 /test-dump-cache-tree
 /test-genrandom
 /test-index-version
+/test-line-buffer
 /test-match-trees
 /test-obj-pool
 /test-parse-options