Improve and clarify branch sorting.

[dmitshur]

Minor issue. I've noticed that some implementations of Branches sort the branches before returning:

sort.Sort(vcs.Branches(bs))

While others do not. The interface does not specify any sorting behavior.

We should improve docs to specify some behavior, and make the output consistent (if it's not).

[dmitshur]

A data point.

Previously, I thought sorting by name was the only reasonable way to sort branch names.

Recently, I've had the need to sort branches by the date of their latest commit.

This makes me think it may be better not to sort branches and let the user do it based on their needs. We could provide helpers for common sort types:

// Sort by branch name.
sort.Sort(vcs.ByName(branches))

// Sort by author date, most recent first.
sort.Sort(sort.Reverse(vcs.ByAuthorDate(branches)))

On the other hand, sorting branches by name may be a decent and inexpensive "default", so perhaps it's fine to always do that (the user can sort again differently if they need).

[sqs]

I think it'd be nice to sort them by ahead and behind, with the default branch in the middle.

like

• a
• b
• master
• c
• d

where a and b are ahead of master and c and d are behind. Not sure how this works with nonlinear history.