nmbug: Translate to Python

This allows us to capture stdout and stderr separately, and do other
explicit subprocess manipulation without resorting to external
packages.  It should be compatible with Python 2.7 and later
(including the 3.x series).

Most of the user-facing interface is the same, but there are a few
changes, where reproducing the original interface was too difficult or
I saw a change to make the underlying Git UI accessible:

* 'nmbug help' has been split between the general 'nmbug --help' and
  the command-specific 'nmbug COMMAND --help'.

* Commands are no longer split into "most common", "other useful", and
  "less common" sets.  If we need something like this, I'd prefer
  workflow examples highlighting common commands in the module
  docstring (available with 'nmbug --help').

* 'nmbug commit' now only uses a single argument for the optional
  commit-message text.  I wanted to expose more of the underlying 'git
  commit' UI, since I personally like to write my commit messages in
  an editor with the notes added by 'git commit -v' to jog my memory.
  Unfortunately, we're using 'git commit-tree' instead of 'git
  commit', and commit-tree is too low-level for editor-launching.  I'd
  be interested in rewriting commit() to use 'git commit', but that
  seemed like it was outside the scope of this rewrite.  So I'm not
  supporting all of Git's commit syntax in this patch, but I can at
  least match 'git commit -m MESSAGE' in requiring command-line commit
  messages to be a single argument.

* The default repository for 'nmbug push' and 'nmbug fetch' is now the
  current branch's upstream (branch.<name>.remote) instead of
  'origin'.  When we have to, we extract this remote by hand, but
  where possible we just call the Git command without a repository
  argument, and leave it to Git to figure out the default.

* 'nmbug push' accepts multiple refspecs if you want to explicitly
  specify what to push.  Otherwise, the refspec(s) pushed depend on
  push.default.  The Perl version hardcoded 'master' as the pushed
  refspec.

* 'nmbug pull' defaults to the current branch's upstream
  (branch.<name>.remote and branch.<name>.merge) instead of hardcoding
  'origin' and 'master'.  It also supports multiple refspecs if for
  some crazy reason you need an octopus merge (but mostly to avoid
  breaking consistency with 'git pull').

* 'nmbug log' now execs 'git log', as there's no need to keep the
  Python process around once we've launched Git there.

* 'nmbug status' now catches stderr, and doesn't print errors like:

    No upstream configured for branch 'master'

  The Perl implementation had just learned to avoid crashing on that
  case, but wasn't yet catching the dying subprocess's stderr.

* 'nmbug archive' now accepts positional arguments for the tree-ish
  and additional 'git archive' options.  For example, you can run:

    $ nmbug archive HEAD -- --format tar.gz

  I wish I could have preserved the argument order from 'git archive'
  (with the tree-ish at the end), but I'm not sure how to make
  argparse accept arbitrary possitional arguments (some of which take
  arguments).  Flipping the order to put the tree-ish first seemed
  easiest.

* 'nmbug merge' and 'pull' no longer checkout HEAD before running
  their command, because blindly clobbering the index seems overly
  risky.

* In order to avoid creating a dirty index, 'nmbug commit' now uses
  the default index (instead of nmbug.index) for composing the commit.
  That way the index matches the committed tree.  To avoid leaving a
  broken index after a failed commit, I've wrapped the whole thing in
  a try/except block that resets the index to match the pre-commit
  treeish on errors.  That means that 'nmbug commit' will ignore
  anything you've cached in the index via direct Git calls, and you'll
  either end up with an index matching your notmuch tags and the new
  HEAD (after a successful commit) or an index matching the original
  HEAD (after a failed commit).

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