commit-graph: error out on invalid commit oids in 'write --stdin-commits'

While 'git commit-graph write --stdin-commits' expects commit object
ids as input, it accepts and silently skips over any invalid commit
object ids, and still exits with success:

  # nonsense
  $ echo not-a-commit-oid | git commit-graph write --stdin-commits
  $ echo $?
  0
  # sometimes I forgot that refs are not good...
  $ echo HEAD | git commit-graph write --stdin-commits
  $ echo $?
  0
  # valid tree OID, but not a commit OID
  $ git rev-parse HEAD^{tree} | git commit-graph write --stdin-commits
  $ echo $?
  0
  $ ls -l .git/objects/info/commit-graph
  ls: cannot access '.git/objects/info/commit-graph': No such file or directory

Check that all input records are indeed valid commit object ids and
return with error otherwise, the same way '--stdin-packs' handles
invalid input; see e103f7276f (commit-graph: return with errors during
write, 2019-06-12).

Note that it should only return with error when encountering an
invalid commit object id coming from standard input.  However,
'--reachable' uses the same code path to process object ids pointed to
by all refs, and that includes tag object ids as well, which should
still be skipped over.  Therefore add a new flag to 'enum
commit_graph_write_flags' and a corresponding field to 'struct
write_commit_graph_context', so we can differentiate between those two
cases.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -74,7 +74,9 @@ int generation_numbers_enabled(struct repository *r);
 enum commit_graph_write_flags {
 	COMMIT_GRAPH_WRITE_APPEND     = (1 << 0),
 	COMMIT_GRAPH_WRITE_PROGRESS   = (1 << 1),
-	COMMIT_GRAPH_WRITE_SPLIT      = (1 << 2)
+	COMMIT_GRAPH_WRITE_SPLIT      = (1 << 2),
+	/* Make sure that each OID in the input is a valid commit OID. */
+	COMMIT_GRAPH_WRITE_CHECK_OIDS = (1 << 3)
 };
 
 struct split_commit_graph_opts {