builtin/reflog: implement subcommand to write new entries

builtin/reflog: implement subcommand to write new entries

While we provide a couple of subcommands in git-reflog(1) to remove
reflog entries, we don't provide any to write new entries. Obviously
this is not an operation that really would be needed for many use cases
out there, or otherwise people would have complained that such a command
does not exist yet. But the introduction of the "reftable" backend
changes the picture a bit, as it is now basically impossible to manually
append a reflog entry if one wanted to do so due to the binary format.

Plug this gap by introducing a simple "write" subcommand. For now, all
this command does is to append a single new reflog entry with the given
object IDs and message to the reflog. More specifically, it is not yet
possible to:

  - Write multiple reflog entries at once.

  - Insert reflog entries at arbitrary indices.

  - Specify the date of the reflog entry.

  - Insert reflog entries that refer to nonexistent objects.

If required, those features can be added at a future point in time. For
now though, the new command aims to fulfill the most basic use cases
while being as strict as possible when it comes to verifying parameters.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>