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.

configitems: support callable as a default value Yuya pointed out that using mutable value as the default could be problematic. To work around this we now support callable object as default value. This allows for creating new mutable objects on demand when needed.

obsutil: move 'getmarkers' to the new modules We have a new 'obsutil' module now. We move the high level utility there to bring 'obsolete.py' back to a more reasonable size.