lock_ref_for_update(): don't re-read non-symbolic references

lock_ref_for_update(): don't re-read non-symbolic references

Before the previous patch, our first read of the reference happened
before the reference was locked, so we couldn't trust its value and had
to read it again. But now that our first read of the reference happens
after acquiring the lock, there is no need to read it a second time. So
move the read_ref_full() call into the (update->type & REF_ISSYMREF)
block.

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