ACE+TAO Debian packaging moved to git

The Git Object Model

It's described with pictures
here.
You really need to understand this if you haven't already.

Once you do, you understand that git is built bottom-up: the plumbing
then the porcelain. If you can't find the tool you need, it's easy to write
it.

git fast-import

The Migrating to Git chapter
explains how you can use the git fast-import tool to manually
import anything into git.

I've used it to create tags with dates in the past, slightly changing the
Custom Importer example in the book:

#!/usr/bin/env ruby
#
# retag.rb
#
# Small script to create an annotated tag, specifying commiter as well as
# date, and tag comment.
#
# Based on Scott Chacon "Custom Importer" example.
#
# Arguments:
#  $1 -- tag name
#  $2 -- sha-1 revision to tag
#  $3 -- committer in the form First Last <email>
#  $4 -- date to use in the form YYYY/MM/DD_HH:MM:SS

def help
  puts "Usage: retag <tag> <sha1sum> <committer> <date> <comment>"
  puts "Creates a annotated tag with name <tag> for commit <sha1sum>, using "
  puts "given <committer>, <date> and <comment>"
  puts "The output should be piped to git fast-import"
end

def to_date(datetime)
  (date, time) = datetime.split('_')
  (year, month, day) = date.split('/')
  (hour, minute, second) = time.split(':')
  return Time.local(year, month, day, hour, minute, second).to_i
end

def generate_tag(tag, sha1hash, committer, date, message)
  puts "tag #{tag}"
  puts "from #{sha1hash}"
  puts "tagger #{committer} #{date} +0000"
  print "data #{message.size}\n#{message}"
end

if ARGV.length != 5
  help
  exit 1
else
  (tag, sha1sum, committer, date, message) = ARGV
  generate_tag(tag, sha1sum, committer, to_date(date), message)
end

graft points

(graft means greffe in French)

Because of missing svn:mergeinfo some changes appear unmerged.
To fix this there are
graft points: they
override git idea of parents of a commit.

To create a graft point, assuming 6a6d48814d0746fa4c9f6869bd8d5c3bc3af8242
is the commit you want to change, currently with a single parent
898ad49b61d4d8d5dc4072351037e2c8ade1ab68, but containing changes from
commit 11cf74d4aa996ffed7c07157fe0780ec2224c73e:

me@mymachine$ echo 6a6d48814d0746fa4c9f6869bd8d5c3bc3af8242 11cf74d4aa996ffed7c07157fe0780ec2224c73e 898ad49b61d4d8d5dc4072351037e2c8ade1ab68 >> .git/info/grafts

git filter-branch

git filter-branch allows you to completely rewrite history of a git branch,
changing or dropping commits while traversing the branch.

As an additional benefit, this tool use graft points and make them permanent.
In other words: after running git filter-branch you can remove
.git/info/grafts file.

I've used it to rewrite author of a given set of commits, using a hack on top of
Chris Johnsen script:

#!/bin/sh

br="HEAD"

TARG_NAME="Raphael Bossek"
TARG_EMAIL="hidden"
export TARG_NAME TARG_EMAIL

filt='

    if test "$GIT_COMMIT" = 546db1966133737930350a098057c4d563b1acdf -o \
            "$GIT_COMMIT" = 23419dde50662852cfbd2edde9468beb29a9ddcc; then
        if test -n "$TARG_EMAIL"; then
            GIT_AUTHOR_EMAIL="$TARG_EMAIL"
            export GIT_AUTHOR_EMAIL
        else
            unset GIT_AUTHOR_EMAIL
        fi
        if test -n "$TARG_NAME"; then
            GIT_AUTHOR_NAME="$TARG_NAME"
            export GIT_AUTHOR_NAME
        else
            unset GIT_AUTHOR_NAME
        fi
    fi

'

git filter-branch $force --tag-name-filter cat --env-filter "$filt" -- $br

(Script edited here; there were much more commits written by Raphael.)

The --tag-name-filter cat argument ensures our tags are copied during the
traversal; otherwise they would be untouched, and hence not available in the
new history.