log: fix behavior with empty repositories (issue3497) Make output in this special case consistent with general case one.

merge with crew

revlog: don't cross-check ancestor result against Python version

parsers: a C implementation of the new ancestors algorithm The performance of both the old and new Python ancestor algorithms depends on the number of revs they need to traverse. Although the new algorithm performs far better than the old when revs are numerically and topologically close, both algorithms become slow under other circumstances, taking up to 1.8 seconds to give answers in a Linux kernel repo. This C implementation of the new algorithm is a fairly straightforward transliteration. The only corner case of interest is that it raises an OverflowError if the number of GCA candidates found during the first pass is greater than 24, to avoid the dual perils of fixnum overflow and trying to allocate too much memory. (If this exception is raised, the Python implementation is used instead.) Performance numbers are good: in a Linux kernel repo, time for "hg debugancestors" on two distant revs (24bf01de7537 and c2a8808f5943) is as follows: Old Python: 0.36 sec New Python: 0.42 sec New C: 0.02 sec For a case where the new algorithm should perform well: Old Python: 1.84 sec New Python: 0.07 sec New C: measures as zero when using --time (This commit includes a paranoid cross-check to ensure that the Python and C implementations give identical answers. The above performance numbers were measured with that check disabled.)

revlog: choose a consistent ancestor when there's a tie Previously, we chose a rev based on numeric ordering, which could cause "the same merge" in topologically identical but numerically different repos to choose different merge bases. We now choose the lexically least node; this is stable across different revlog orderings.

ancestor: a new algorithm that is faster for nodes near tip Instead of walking all the way to the root of the DAG, we generate a set of candidate GCA revs, then figure out which ones will win the race to the root (usually without needing to traverse all the way to the root). In the common case of nodes that are close to each other in both revision number and topology, this is usually a big win: it makes "hg --time debugancestors" up to 9 times faster than the more general ancestor function when measured on heads of the linux-2.6 hg repo. Victory is not assured, however. The older function can still win by a large margin if one node is much closer to the root than the other, or by a much smaller amount if one is an ancestor of the other. For now, we've also got a small paranoid harness function that calls both ancestor functions on every input and ensures that they give equivalent answers. Even without the checker function, the old ancestor function needs to stay alive for the time being, as its generality is used by context.filectx.merge.