Add num_commits_to_* methods to Project object

[ChrisTimperley]

This family of methods should be used to calculate the number of commits to a given repository, file, line or function within a given range of commits.

def num_commits_to_repo(...) -> int:
def num_commits_to_file(...) -> int:
def num_commits_to_function(...) -> int:
def num_commits_to_line(...) -> int:

Each of these functions should have a similar header:

def num_commits_to_repo(repo: git.Repo,
                        before: Optional[git.Commit] = None,
                        after: Optional[git.Commit] = None,
                        ...) -> int:

Internally, each of these methods should call commits_to_* method to obtain a set of commits, before returning the length of that list.

def commits_to_repo(...) -> List[git.Commit]
def commits_to_file(...) -> List[git.Commit]
def commits_to_function(...) -> List[git.Commit]
def commits_to_line(...) -> List[git.Commit]

One could argue that the num_commits_to_* methods are superfluous. Either way, we still need to implement commits_to_function and commits_to_repo.

[jlacomis]

This functionality exists now.

[jlacomis]

Whoops, forgot that "commits_to_function" isn't done yet