Create server setting to auto-create branches on connect

[zachmu]

Should be possible to connect to mydb/newBranch without creating that branch first.

[timsehn]

test-commit-diff $ mysql -h 127.0.0.1 --database='test_commit_diff/foo' -u root
ERROR 1105 (HY000): unknown error: database not found: test_commit_diff/foo
test-commit-diff $ mysql -h 127.0.0.1 --database='test_commit_diff/main' -u root
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.7.9-Vitess 

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mysql>

The first command should work and just create the foo branch off HEAD of default branch.

[zachmu]

Slight variation: should be possible to name a remote to automatically pull branches from on connection if they don't exist locally.

This is already the case for read replicas, but would be nice to support on normal clones as well.