rerere: lift PATH_MAX limitation

rerere: lift PATH_MAX limitation

The MERGE_RR file records a collection of NUL-terminated entries,
each of which consists of

 - a hash that identifies the conflict
 - a HT
 - the pathname

We used to read this piece-by-piece, and worse yet, read the
pathname part a byte at a time into a fixed buffer of size PATH_MAX.

Instead, read a whole entry using strbuf_getwholeline() and parse
out the fields.  This way, we issue fewer read(2) calls and more
importantly we do not have to limit the pathname to PATH_MAX.

Signed-off-by: Junio C Hamano <gitster@pobox.com>