Add stream helper library

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 <redacted>
Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>