commit: rewrite read_graft_line

commit: rewrite read_graft_line

Old implementation determined number of hashes by dividing length of
line by length of hash, which works only if all hash representations
have same length.

New graft line parser works in two phases:

  1. In first phase line is scanned to verify correctness and compute
     number of hashes, then graft struct is allocated.

  2. In second phase line is scanned again to fill up already allocated
     graft struct.

This way graft parsing code can support different sizes of hashes
without any further code adaptations.

A number of alternative implementations were considered and discarded:

  - Modifying graft structure to store oid_array instead of FLEXI_ARRAY
    indicates undesirable usage of struct to readers.

  - Parsing into temporary string_list or oid_array complicates code
    by adding more return paths, as these structures needs to be
    cleared before returning from function.

  - Determining number of hashes by counting separators might cause
    maintenance issues, if this function needs to be modified in future
    again.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>