Adds dolt_merge_status system table

[druvv]

Closes #3504

The dolt_merge_status system table tells a user if a merge is active. It has the following schema:

CREATE TABLE `dolt_merge_status` (
-- Whether a merge is currently active or not
  `is_merging` tinyint NOT NULL, 
 -- The commit spec that was used to initiate the merge
  `source` text,
-- The commit that the commit spec resolved to at the time of merge
  `source_commit` text, 
-- The target destination working set
  `target` text, 
-- A list of tables that have conflicts or constraint violations
  `unmerged_tables` text 
)

For example, lets create a simple conflict:

dolt sql -q "CREATE TABLE t (a INT PRIMARY KEY, b INT);"
dolt add .
dolt commit -am "base"

dolt checkout -b right
dolt sql <<SQL
ALTER TABLE t ADD c INT;
INSERT INTO t VALUES (1, 2, 1);
SQL
dolt commit -am "right"

dolt checkout main
dolt sql -q "INSERT INTO t values (1, 3);"
dolt commit -am "left"

dolt merge right

Output of SELECT * from dolt_merge_status;:

+------------+--------+----------------------------------+-----------------+-----------------+
| is_merging | source | source_commit                    | target          | unmerged_tables |
+------------+--------+----------------------------------+-----------------+-----------------+
| true       | right  | fbghslue1k9cfgbi00ti4r8417frgbca | refs/heads/main | t               |
+------------+--------+----------------------------------+-----------------+-----------------+

[zachmu]

Some questions about this specific design. Seems like a row per unmerged table might be a bit more ergonomic?

[zachmu]

Is this actually dolt_unmerged_tables, with one row per unmerged table?

No rows if no unmerged tables?

Get rid of is_merging and replace with a merge_active() function?

[druvv]

A flat design could work here as well, but determining which tables are unmerged isn't the primary use case. The main reason we want a merge status function is to know if we need to commit a merge or if we need to abort the active one.

The unmerged tables column was added for more similarity between dolt status and this new procedure.

[zachmu]

Isn't this just active_branch()?

[druvv]

Correct, target is just the active branch. Do you think we should name it differently than target?

[zachmu]

Need some tests with more than one table unmerged