refs.c: rename log_ref_setup to create_reflog

Change the name from log_ref_setup to create_reflog. Make create_reflog
create the reflog unconditionally, whatever log_all_ref_updates is set to,
and instead check for log_all_ref_updates before calling it in log_ref_write.

Sicne create_reflog no lonber checks log_all_ref_updates we can skip
temporarily setting it to true before calling create_reflog from
checkout.c.

log_ref_setup/create_reflog is relatively expensive, making many system
calls, and the common case will be that the reflog already exists.
Therefore use the much cheaper reflog_exists to check and only call
create_reflog if we actually do need to create the reflog when calling from
log_ref_write.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: allow passing raw git_committer_info as email to _update_reflog

In many places in the code we do not have access to the individual fields
in the committer data. Instead we might only have access to prebaked data
such as what is returned by git_committer_info() containing a string
that consists of email, timestamp, zone etc.

This makes it inconvenient to use transaction_update_reflog since it means you
would have to first parse git_committer_info before you can call update_reflog.

Add a new flag REFLOG_EMAIL_IS_COMMITTER to _update_reflog to tell it
that what we pass in as email is already the fully baked committer string
we can use as-is.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>

Conflicts:
	refs.c
refs.c: return error instead of dying when locking fails during transaction

Change lock_ref_sha1_basic to return an error instead of dying when
we fail to lock a file during a transaction.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: use packed refs when deleting refs during a transaction

Make the deletion of refs during a transaction more atomic.
Start by first copying all loose refs we will be deleting to the packed
refs file and then commit the packed refs file. Then re-lock the packed refs
file to avoid anyone else from modifying the refs we are to delete during this
transaction and keep it locked until we are finished.
Since all refs we are about to delete are now safely held in the packed refs
file we can proceed to immediately unlink any loose refs for those refs
and still be fully rollback-able.

The exception is for refs that can not be resolved. Those refs are never
added to the packed refs and will just be un-rollback-ably deleted during
commit.

By deleting all the loose refs at the start of the transaction we make make it
possible to both delete one ref and then re-create a different ref in the
same transaction, even if both the old-to-be-deleted and the new-to-be-created
refs would otherwise conflict in the file-system.

Example: rename m->m/m

In that example we want to delete the file 'm' so that we make room so
that we can create a directory with the same name in order to lock and
write to the ref m/m and its lock-file m/m.lock.

If there is a failure during the commit phase we can rollback without losing
any refs since we have so far only deleted the loose refs but we still have
the refs stored in the packed refs file.
Once we have finished all updates for refs and their reflogs we can repack
the packed refs file and remove the to-be-deleted refs from the packed refs,
at which point all the deleted refs will disappear in one atomic rename
operation.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: update rename_ref to use a transaction

Change refs.c to use a single transaction to copy/rename both the refs and
its reflog. Since we are no longer using rename() to move the reflog file
we no longer need to disallow rename_ref for refs with a symlink for its reflog
so we can remove that test from the testsuite.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>