Duplicate ignore rule methods

[josharian]

Repository methods include:

func (r *Repository) AddGitIgnoreRules(rules string) error
func (v *Repository) AddIgnoreRule(rules string) error
func (r *Repository) ClearGitIgnoreRules() error
func (v *Repository) ClearInternalIgnoreRules() error

The implementation is almost identical.

I suggest marking one of each of the pairs as deprecated, and having it just call through to its sibling.