Allow passing a Reference to Repository.merge()

[jorio]

This PR allows passing more types to Repository.merge. Previously, you could only pass a commit ID as an Oid or an str.

In addition to these two, you can now also pass a real Commit, or a Reference.

Passing in a Reference will enrich the merge with additional information. For example, Repository.message will specify the name of the branch being merged, instead of just stating a commit ID:

branch = repo.branches.local['other']
repo.merge(branch)
print(repo.message)  # "Merging branch 'other'"

If this PR is accepted, I believe passing an str should now be deprecated. The str is supposed to represent a commit ID, but users might intuit that the function also accepts ref names as an str, when in fact they should pass a real Reference object.

[jdavid]

If this PR is accepted, I believe passing an str should now be deprecated. The str is supposed to represent a commit ID, but users might intuit that the function also accepts ref names as an str, when in fact they should pass a real Reference object.

Agree, there should be a deprecation warning, could you make a PR?