[PATCH RFC 0/4] rebase -i: Add --refs option to rewrite heads within branch

This RFC series adds a "ref" command to rebase -i, and an option --refs to generate "ref" commands in the original TODO file. This makes it easy to rebase a branch together with intermediate markers, or a series of branches.

For example, in this situation:

      part1 part2 topic
        |     |     |
        v     v     v
  A--*--*--*--*--*--*
   \
    B <--master

we may want to rebase 'topic' onto the new master, rewriting the intermediate branches 'part1', 'part2' to the corresponding new commits. This can be done with a sequence of "git rebase --onto" commands, but it's tricky to get right.

With this series, the command
  $ git rebase -i --refs master topic
suffices to produce this result:

        part1 part2 topic
  A       |     |     |
   \      v     v     v
    B--*--*--*--*--*--*
    ^
    |
    master

These patches work for me.  Before I recommend the last two patches
for merge, they'll need the following additional work:
 - interoperate with rebase --abort
 - add documentation
 - probably add tests

The series begins with two patches that are independently useful, and
that I believe are ready for merge:
  pretty: Add %D for script-friendly decoration
  log --decorate=full: drop the "tag: " prefix
They make the functionality of 'git log --decorate' available in an
output format optimized for machine parsing.  I expect other people
will find this useful for other scripts.

Greg