graft: record intermediate grafts in extras Previously the extra field for a graft only contained the original commit hash. This made it impossible to use graft to copy a commit more than once, because the extras fields did not change after the second graft. The fix is to add an extra.intermediate-source field that records the immediate predecessor to graft. This changes hashes for commits that have been grafted twice, which is why the test was affected.

graft: allow creating sibling grafts Previously it was impossible to graft a commit onto it's own parent (i.e. create a copy of the commit). This is useful when wanting to create a backup of the commit before continuing to amend it. This patch enables that behavior. The change to the histedit test is because histedit uses graft to apply commits. The test in question moves a commit backwards onto an ancestor. Since the graft logic now more explicitly supports this, it knows to simply accept the incoming changes (since they are more recent), instead of prompting.

unbundle20: move header parsing into the 'getunbundler' function The dispatching will be based on the header content, so we need to move this logic into the factory function.