Write to the HEAD reflog when renaming the currently active branch

[jgrosso]

Resolves #4292.

[tiennou]

Here are the lines added to the HEAD reflog after the test executes.

0: 000000000000000000000000000000000000000 a65fedf39aefe402d3bb6e24df4d4f5fe454775 somesig checkout: moving from master to renamed
1: 000000000000000000000000000000000000000 a65fedf39aefe402d3bb6e24df4d4f5fe454775 somesig (null)

1 is the message I'm passing when renaming, 0 is the result of the git_repository_set_head call.

So "in theory" it works : the message when renaming now appears in the HEAD reflog. But it doesn't match what git does.

[ethomson]

Closing as stale — we need to always match what git does