Drop strbuf's 'eof' marker, and make read_line a first class citizen.

Drop strbuf's 'eof' marker, and make read_line a first class citizen.

read_line is now strbuf_getline, and is a first class citizen, it returns 0
when reading a line worked, EOF else.

The ->eof marker was used non-locally by fast-import.c, mimic the same
behaviour using a static int in "read_next_command", that now returns -1 on
EOF, and avoids to call strbuf_getline when it's in EOF state.

Also no longer automagically strbuf_release the buffer, it's counter
intuitive and breaks fast-import in a very subtle way.

Note: being at EOF implies that command_buf.len == 0.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>