log: add an extension hook-point in changeset_printer Extensions sometimes wants to add other information in the default log output format (when no templating is used). Add an empty function named '_exthook' for easing the extension life. Extensions will be able to wrap this function and collaborate to display additional information. Exthook is called after displaying troubles and just before displaying the files, extra and description. Add a new test file to test it and not pollute other test files.

drawdag: support obsmarker creation in comments This patch adds special comment handling so one can create obsmarkers in drawdag comments like "# replace: A -> B -> C", "# prune: X, Y, Z", "split: P -> M, N" and they are just self-explained.

test-drawdag: add a test for drawdag.py The drawdag script is useful but does not have a formal test. This patch adds it.

filemerge: convert a couple of wvfs calls in internal mergetools to contexts One hitch is that sometimes fcd is actually an absentfilectx which does not expose any mutator functions. In order to still use the context functions, we look up the underlying workingfilectx to perform the write there. One alternate way would be to put the write functions on the absentfilectx and have them pass-through. While this makes the callsites cleaner, we would need to decide what its getter functions would return after this point, since returning None for `data` (and True for `isabsent()`) might no longer be correct after a write. I discussed with Sidd about just having the getters raise RuntimeErrors after a mutator has been called, but we actually call isabsent() in merge.py after running the internal merge tools.