libre2c: compare histories in a trie without reconstructing them.

libre2c: compare histories in a trie without reconstructing them.

Previously we used to unwing each history, store it as a linear sequence
in a separate buffer, and then iterate buffers from the beginning of
histories to fing the fork point.

This reconstruction step can be avoided, because we can find fork by
tracing both histories together from tail to root, element by element,
until their indices coincide (this is the fork point. Someone called
thos "two-fingers algorithm".