"git log" learned the "--author-date-order" option, with which the
output is topologically sorted and commits in parallel histories
are shown intermixed together based on the author timestamp.
* jc/topo-author-date-sort:
t6003: add --author-date-order test
topology tests: teach a helper to set author dates as well
t6003: add --date-order test
topology tests: teach a helper to take abbreviated timestamps
t/lib-t6000: style fixes
log: --author-date-order
sort-in-topological-order: use prio-queue
prio-queue: priority queue of pointers to structs
toposort: rename "lifo" field