files-backend: extract out `create_symref_lock()`

files-backend: extract out `create_symref_lock()`

The function `create_symref_locked()` creates a symref by creating a
'<symref>.lock' file and then committing the symref lock, which creates
the final symref.

Extract the early half of `create_symref_locked()` into a new helper
function `create_symref_lock()`. Because the name of the new function is
too similar to the original, rename the original to
`create_and_commit_symref()` to avoid confusion.

The new function `create_symref_locked()` can be used to create the
symref lock in a separate step from that of committing it. This allows
to add transactional support for symrefs, where the lock would be
created in the preparation step and the lock would be committed in the
finish step.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>