Fix reloading of references to not cause access to freed memory

Fix reloading of references to not cause access to freed memory

Make the allocation of refs stable across reloads (of either the main,
branch or log view) by changing the storage method and introducing a
struct ref_list to keep track of lists of references.

read_ref now always scans the already allocated refs. To speed this up
keep the list sorted and use binary search when inserting and updating.