bundle: add optional 'tagsfnodecache' data to on disk bundle (issue5543) This should help performance when unbundling.

bundle2: move tagsfnodecache generation in a generic function This will help us reusing the logic for `hg bundle`.

bundle: introduce an higher level function to write bundle on disk The current function ('writebundle') is focussing on getting an existing changegroup to disk. It is no easy ways to includes more part in the generated bundle2. So we introduce a slightly higher level function that is fed the 'outgoing' object (that defines the bundled spec) and the bundlespec parameters (to control the changegroup generation and inclusion of other parts). This is creating the third logic dedicated to create a consistent bundle2 (the other 2 are the push code and the getbundle code). We should probably reconcile them at some points but they all takes different types of input. So we need to introduce an intermediate "object" that each different input could be converted to. Such unified "bundle2 specification" could be fed to some unified code. We start by having the `hg bundle` related code on its own to helps defines its specific needs first. Once the common and specific parts of each logic will be known we can start unification.

bundle: handle compression earlier We can also handle that part before starting any generation.

bundle: check changegroup version earlier We can check if we know how to bundle this changegroup version before actually starting to generate the changegroup.

bundle: check lack of revs to bundle before generating the changegroup We already have the information so we can check it earlier.

extdiff: copy back files to the working directory if the size changed In theory, it should be enough to pay attention only to the modification time when detecting if a snapshotted working directory file changed. In practice, BeyondCompare preserves all file attributes when syncing files at the directory level. (If you open the file and sync individual hunks, then mtime does change, and everything was being copied back as desired.) I'm not sure how many other synchronization tools would trigger this issue, but it's annoyingly inconsistent (if a single file is diffed, it isn't snapshotted, so the same BeyondCompare file sync operation _is_ visible, because wdir() is updated in place. I filed a bug with them, and they stated it is on their wish list, but won't be fixed in the near term. This isn't a complete fix (there is still the case of the size not changing), but this seems like a trivial enough change to fix most of the problem. I suppose we could fool around with making files in the other snapshot readonly, and copy back if we see the readonly bit copied. That seems pretty hacky though, and only works if the external tool copies all attributes.