Fix JSON merge issue that would report imprecise diffs in some situations

[nicktobey]

If:

• A chunk begins with an object "A"
• A value "A.b" within this object was modified
• The previous chunk was also modified

Then the differ would incorrectly report that the entire "A" object had been modified, instead of the sub-value "A.b"

This could then lead to situations where the merge cannot be auto-resolved, because the differ claims that an object has been modified divergently, when it's actually two unrelated fields in the object that have been modified.

This PR fixes that scenario by properly detecting when the next chunk marks the start of an object.

[coffeegoddd]

@nicktobey DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
c9b6269	ok	5937457

version	total_tests
c9b6269	5937457

correctness_percentage
100.0

[github-actions[bot]]

@coffeegoddd DOLT	test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.06	1.33
batching	batch sql	10000	1	0.08	1.5
batching	by line sql	10000	1	0.07	1.71
blob	1 blob	200000	1	0.87	3.86	3.68
blob	2 blobs	200000	1	0.87	4.37	4.49
blob	no blob	200000	1	0.89	2.35	2.02
col type	datetime	200000	1	0.81	2.93	2.81
col type	varchar	200000	1	0.65	3.48	2.91
config width	2 cols	200000	1	0.81	2.31	1.94
config width	32 cols	200000	1	1.84	1.99	2.29
config width	8 cols	200000	1	0.99	2.23	1.95
pk type	float	200000	1	1.09	1.73	1.45
pk type	int	200000	1	1.07	1.75	1.41
pk type	varchar	200000	1	1.84	1.41	1.12
row count	1.6mm	1600000	1	5.64	2.81	2.28
row count	400k	400000	1	1.44	2.69	2.19
row count	800k	800000	1	2.83	2.75	2.25
secondary index	four index	200000	1	3.55	1.35	1.05
secondary index	no secondary	200000	1	0.89	2.29	2.04
secondary index	one index	200000	1	1.13	2.28	1.93
secondary index	two index	200000	1	1.96	1.7	1.39
sorting	shuffled 1mm	1000000	0	5.31	2.68	2.35
sorting	sorted 1mm	1000000	1	5.42	2.72	2.25

[github-actions[bot]]

@coffeegoddd DOLT	name	detail	mean_mult
dolt_blame_basic	system table	1.13
dolt_blame_commit_filter	system table	2.87
dolt_commit_ancestors_commit_filter	system table	0.58
dolt_commits_commit_filter	system table	1.05
dolt_diff_log_join_from_commit	system table	2.06
dolt_diff_log_join_to_commit	system table	2.08
dolt_diff_table_from_commit_filter	system table	1.2
dolt_diff_table_to_commit_filter	system table	1.21
dolt_diffs_commit_filter	system table	1
dolt_history_commit_filter	system table	1.42
dolt_log_commit_filter	system table	1.17

[github-actions[bot]]

@coffeegoddd DOLT	name	add_cnt	delete_cnt	update_cnt	latency
adds_only	60000	0	0	0.72
adds_updates_deletes	60000	60000	60000	3.79
deletes_only	0	60000	0	1.84
updates_only	0	0	60000	2.43