[PATCH 0/5] Modernize read_graft_line implementation

[Patryk Obara <patryk.obara@gmail.com>]

I experimented with using a different hash algorithm (I am aware of
existing "Git hash function transition plan", I just want to push
things forward a bit) - and immediately hit a small issue - changing
the size of object_id hash buffer leads to compilation issues and
breaks graft-related tests.

I am sending patch 1 only to show a modification, that I did to
increase buffer size - it's not intended to be merged.

Patch 2 fixes trivial compilation issue.

Patches 3, 4, and 5 touch graft implementation to remove calculations
using GIT_SHA1_*, that lead to broken tests. I replaced FLEX_ARRAY of
object_id's representing parents with oid_array. New implementation
should be more future-proof, I think.

New implementation has tiny behaviour change: previously parents in
graft line needed to be separated with single space - now any number
of whitespace characters will do.

Alternative implementation approaches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Strbuf could be replaced with string_list with
string_list_split_in_place instead of while loop in read_graft_line.
I didn't implement it this way because I learned
about string_list_split_in_place after finishing this implementation
draft. Right now I'm not sure which approach is better.

Another possibility is dropping graft feature altogether - that would
mean removing code for parsing grafts and 'parent' field in the struct,
but preserving the struct itself as a shallow clone marker. Grafts are
a little-known feature with modern replacement, but this seems like
bigger task and rather out of the scope of transition to the new
hashing algorithm.

I considered making function read_graft_line a static one and
read_graft_file non-static, but read_graft_line is used in
'builtin/blame.c' in function read_ancestry, which is almost a copy of
read_graft_file (difference of single boolean flag passed to
register_commit_graft). Removal of this duplication may be worthwhile,
but I think it's out of scope.

Patryk Obara (5):
  cache: extend object_id size to sha3-256
  sha1_file: fix hardcoded size in null_sha1
  commit: replace the raw buffer with strbuf in read_graft_line
  commit: implement free_commit_graft
  commit: rewrite read_graft_line

 builtin/blame.c |  2 +-
 cache.h         |  8 ++++++--
 commit.c        | 55 ++++++++++++++++++++++++++++++++-----------------------
 commit.h        |  5 +++--
 sha1_file.c     |  2 +-
 shallow.c       |  1 +
 6 files changed, 44 insertions(+), 29 deletions(-)

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2.9.5