vcs-svn: allow character-oriented input

vcs-svn: allow character-oriented input

buffer_read_char can be used in place of buffer_read_string(1) to
avoid consuming valuable static buffer space.  The delta applier will
use this to read variable-length integers one byte at a time.

Underneath, it is fgetc, wrapped so the line_buffer library can
maintain its role as gatekeeper of input.

Later it might be worth checking if fgetc_unlocked is faster ---
most line_buffer functions are not thread-safe anyway.

Helpd-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>