refs: add a concept of a reference transaction

Build out the API for dealing with a bunch of reference checks and
changes within a transaction.  Define an opaque ref_transaction type
that is managed entirely within refs.c.  Introduce functions for
beginning a transaction, adding updates to a transaction, and
committing/rolling back a transaction.

This API will soon replace update_refs().

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
update-ref --stdin: reimplement using reference transactions

This change is mostly clerical: the parse_cmd_*() functions need to
use local variables rather than a struct ref_update to collect the
arguments needed for each update, and then call ref_transaction_*() to
queue the change rather than building up the list of changes at the
caller side.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
refs: remove API function update_refs()

It has been superseded by reference transactions.  This also means
that struct ref_update can become private.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
struct ref_update: rename field "ref_name" to "refname"

This is consistent with the usual nomenclature.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
struct ref_update: store refname as a FLEX_ARRAY

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
ref_transaction_commit(): simplify code using temporary variables

Use temporary variables in the for-loop blocks to simplify expressions
in the rest of the loop.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
struct ref_update: add a lock field

Now that we manage ref_update objects internally, we can use them to
hold some of the scratch space we need when actually carrying out the
updates.  Store the (struct ref_lock *) there.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
struct ref_update: add a type field

It used to be that ref_transaction_commit() allocated a temporary
array to hold the types of references while it is working.  Instead,
add a type field to ref_update that ref_transaction_commit() can use
as its scratch space.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
ref_transaction_commit(): work with transaction->updates in place

Now that we free the transaction when we are done, there is no need to
make a copy of transaction->updates before working with it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>