crecord: call prevsibling() and nextsibling() directly The 3 classes for items used in crecord (uiheader, uihunk, uihunkline) all have prevsibling() and nextsibling() methods. The two methods are used to get the previous/next item of the same type of the same parent element as the current one: when `a` is a uihunkline instance, a.nextsibling() returns the next line in this hunk (or None, if `a` is the last line). There are also two similar methods: previtem() and nextitem(). When called with constrainlevel=True (the default) they simply returned the result of prevsibling()/nextsibling(). Only when called with constrainlevel=False they did something different: they returned previous/next item regardless of its type (so if `a` is the last line in a hunk, a.nextitem(constrainlevel=False) could return the next hunk or the next file -- something that is not a line). Let's simplify this logic and make code call -sibling() methods when only siblings are needed and -item() methods when any item would do, and then remove the constrainlevel argument from previtem() and nextitem().

dispatch: add fail-* family of hooks The post-* family of hooks will not run in case a command fails (i.e. raises an exception). This makes it inconvenient to hook into events such as doing something in case of a failed push. We catch all exceptions to run the failure hook. I am not sure if this is too aggressive, but tests apparently pass.

py3: make hgext/rebase.py use absolute_import