Faithfully convert Subversion repositories to Git.

SubConvert.py, version 1.0
 
Copyright (c) 2011, BoostPro Computing.  All rights reserved.

This script replays Subversion dump files as Git actions, yielding a Git
repository which has as close of a 1:1 correspondence with the original
Subversion repository as possible.

Some respects in which this are not possible are:

* Subversion allows for multiple transactions within a single revision, and it
  is possible that some of those transactions may affect more than one branch.
  This must be mapped as two separate Git commits if/when it occurs.

* Subversion supports revisions which modify only directories and/or
  properties of files and directories.  Since Git tracks only files, and has
  no notion of Subversion's properties, these revisions are ignored.

* Subversion models all content in a flat filesystem, such that semantically,
  there is no distinction between branches and tags, except that typically a
  "tag" is a directory which is never modified after initial creation.

  Because proper identification of branches and tags cannot faithfully be done
  hueristically, this script makes a best guess based on activity within all
  revisions, and then outputs a data file for the user to correct before
  performing the final conversion.

* Subversion also tracks version history for multiple projects within this
  same, single filesystem.  This script, if provided with a submodules
  "manifest" file, can create multiple repositories in parallel: one to model
  the original Subversion repository as exactly as possible, with all projects
  conflated in a single filesystem; and a separate repository for each
  submodule.

Note that for efficiency's sake -- to avoid thrashing disk unnecessarily and
taking orders of magnitude more time -- this script performs Git actions
directly, completely bypassing use of a working tree.  That is, instead of
using porcelain commands such as git add, remove, commit, etc., and git
checkout to switch between branches, it uses the underlying plumbing commands:
hash-object, mktree, commit-tree, update-ref, symbolic-ref, etc.  The final
checkout to yield the working tree(s) is done only after all repositories and
their branches and tags have been finalized.