Bug: base manifest value for 'exclude' is always ignored #227

I stumbled upon this bug while working on #226: the culprit is Repository.hooks.

A quick fix for this would be to simply remove the default value from pre_commit/clientlib/validate_config.py, but the root cause is that any default value defined for a field in this file will make the corresponding manifest field useless.

Basically here is what happens in Repository.hooks:

• all the hooks defined in the current repository are enumerated
• at this stage, a hook is a dict closely matching the Yaml the config file content, plus default values for fields not defined in the Yaml but having a JSON schema 'default'
• when doing the dict merge, every (key,value) pair in hook overrides the corresponding manifest entry. This includes default config value like exclude: '$^' overriding a base manifest value like exclude: '.bak$'

Hence I suggest either adding a test ensuring there will never be any 'default' defined in CONFIG_JSON_SCHEMA, or improving the merge logic.