setup.py: don't rewrite @LIBDIR@ when creating wheels This is necessary to produce wheels that install properly. More details are captured in an in-line comment. After this patch, produced wheels can be installed via `pip install` and appear to "just work," including on Windows.

setup.py: attempt to build and install hg.exe on Windows Currently, packaging Mercurial on Windows will produce a Scripts\hg Python script and a Scripts\hg.bat batch script. The py2exe distribution contains a hg.exe which loads a Python interpretter and invokes the "hg" Python script. Running a exe directly has benefits over batch scripts because batch scripts do things like muck around with command arguments. This patch implements a custom "build_scripts" command which attempts to build hg.exe on Windows. If hg.exe is built, it is marked as a "script" file and installed into the Scripts\ directory on Windows. Since hg.exe is redundant and better than hg.bat, if hg.exe is built, hg.bat is not installed. Since some environments don't support compiling C programs, we treat hg.exe as optional and catch failures building it. This is not ideal. However, I reckon most Windows users will not be installing Mercurial from source: they will get it from the MSI installer or via `pip install Mercurial`, which will download a wheel that has hg.exe in it. So, I don't think this is a big deal.

merge.graft: add option to keep second parent Currently merge.graft re-writes the dirstate so only a single parent is kept. For some cases, like evolving a merge commit, this behaviour is not desired. More specifically, this is needed to fix issue4389.

ui: support declaring path push urls as sub-options Power users often want to apply per-path configuration options. For example, they may want to declare an alternate URL for push operations or declare a revset of revisions to push when `hg push` is used (as opposed to attempting to push all revisions by default). This patch establishes the use of sub-options (config options with ":" in the name) to declare additional behavior for paths. New sub-options are declared by using the new ``@ui.pathsuboption`` decorator. This decorator serves multiple purposes: * Declaring which sub-options are registered * Declaring how a sub-option maps to an attribute on ``path`` instances (this is needed to `hg paths` can render sub-options and values properly) * Validation and normalization of config options to attribute values * Allows extensions to declare new sub-options without monkeypatching * Allows extensions to overwrite built-in behavior for sub-option handling As convenient as the new option registration decorator is, extensions (and even core functionality) may still need an additional hook point to perform finalization of path instances. For example, they may wish to validate that multiple options/attributes aren't conflicting with each other. This hook point could be added later, if needed. To prove this new functionality works, we implement the "pushurl" path sub-option. This option declares the URL that `hg push` should use by default. We require that "pushurl" is an actual URL. This requirement might be controversial and could be dropped if there is opposition. However, objectors should read the complicated code in ui.path.__init__ and commands.push for resolving non-URL values before making a judgement. We also don't allow #fragment in the URLs. I intend to introduce a ":pushrev" (or similar) option to define a revset to control which revisions are pushed when "-r <rev>" isn't passed into `hg push`. This is much more powerful than #fragment and I don't think #fragment is useful enough to continue supporting. The [paths] section of the "config" help page has been updated significantly. `hg paths` has been taught to display path sub-options. The docs mention that "default-push" is now deprecated. However, there are several references to it that need to be cleaned up. A large part of this is converting more consumers to the new paths API. This will happen naturally as more path sub-options are added and more and more components need to access them.

ui: pass ui instance to path.__init__ It will be used in a subsequent patch.

ui: store pushloc as separate attribute The magic @property is going to interfere with the ability to print path sub-options. We only access it in one location and it is trivial to in-line, so do that.

commands: add debugdeltachain command We have debug commands for displaying overall revlog statistics (debugrevlog) and for dumping a revlog index (debugindex). As part of investigating various aspects of revlog behavior and performance, I found it important to have an understanding of how revlog delta chains behave in practice. This patch implements a "debugdeltachain" command. For each revision in a revlog, it dumps information about the delta chain. Which delta chain it is part of, length of the delta chain, distance since base revision, info about base revision, size of the delta chain, etc. The generic formatting facility is used, which means we can templatize output and get machine readable output like JSON. This command has already uncovered some weird history in mozilla-central I didn't know about. So I think it's valuable.