rs/ref-transaction-1

"Use ref transactions", early part

Ronnie explains:

	This patch series expands on the transaction API. It converts
	ref updates, inside refs.c as well as external, to use the
	transaction API for updates. This makes most ref updates
	atomic when there are failures locking or writing to a ref.

This series teaches the tag, replace, commit, cherry-pick, fast-import,
checkout -b, branch, receive-pack, and http-fetch commands and all
update_ref and delete_ref callers to use the ref transaction API instead
of lock_ref_sha1.

The main user-visible change should be some cosmetic changes to error
messages.  The series also combines multiple ref updates into a single
transaction in 'git http-fetch -w' and when writing tags in fast-import
but otherwise doesn't change the granularity of transactions.
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