Merge branch 'ty/migrate-excludes-file'

Merge branch 'ty/migrate-excludes-file'

The 'excludes_file' and various other global configuration variables
(including 'editor_program', 'pager_program', 'askpass_program', and
'push_default') have been migrated into the per-repository structure.

* ty/migrate-excludes-file:
  repository: adjust the comment of config_values_private_
  environment: move object_creation_mode into repo_config_values
  environment: move autorebase into repo_config_values
  environment: move push_default into repo_config_values
  environment: migrate apply_default_whitespace and apply_default_ignorewhitespace
  environment: move askpass_program into repo_config_values
  environment: move pager_program into repo_config_values
  environment: move editor_program into repo_config_values
  environment: move excludes_file into repo_config_values
  repository: introduce repo_config_values_clear()