encoding: avoid cyclic dependency around "parsers" in pure Python build 80f2b63dd83a brought "asciilower" and "import parsers" into "encoding.py". This works fine with "parsers" module in C implementation, but doesn't with one in pure Python implementation, because the latter causes cyclic dependency below and aborting execution: util => i18n => encoding => parsers => util This patch delays importing "parsers" module until it is really needed, to avoid cyclic dependency around "parsers" in pure Python build.

pull: add source information to the transaction The source information can, should be applied once when opening the transaction for the pull. This will lets element processed within a bundle2 be aware of them and open the door to running a set of hooks when closing this pull transaction. This is similar to what is done in server's unbundle call.

changegroup: store source and url in the `hookargs` dict We store the source and url of the current data into `transaction.hookargs` this let us inherit it from upper layers that may have created a much wider transaction. We have to modify bundle2 at the same time to register the source and url in the transaction. We have to do it in the same patch otherwise, the `addchangegroup` call would fill these values and the hook calling will crash because of the duplicated 'source' and 'url' arguments passed to the hook call.

prechangegroup: use hook argument from the transaction There can be useful data in there (eg: bundle2 related one)

addchangegroup: call `prechangegroup` hook after transaction retrieval We want to reused some possible information stored in the transaction `hookargs` dict that may be stored by something handling the transaction at an upper level (eg: bundle2) So we move the running of the hooks after transaction creation. This has no visible effects (but an empty transaction roolback if the hook fails) because nothing had happened in the transaction yet.

addchangegroup: get the `node` argument of `incoming` hook from transaction The transaction is now carrying hook-related informations. So we use it to retrieve the `node` argument. This will also carry around all kinds of other useful informations (like: "are we in a bundle2 processing")

dicthelpers: delete now that they are no longer used

manifest: transpose pair of pairs from diff() It makes more sense for the file nodeids and returned from diff() to be ((n1,fl1),(n2,fl2)) than ((n1,n2),(fl1,fl2)), so change it to the former.