SplitGitRepository.ps1

# How-to to copy take a directory 'BuildScripts' out of a big GIT repository 'ContactCentre', retainining history

# STEP 1: Create the BuildScripts repository

# Clone the big repository, which will be stripped down to the directory BuildScripts
git clone ContactCentre BuildScripts

# Safety measure: detach from server, to prevent pushing a split back to the server by mistake
git remote rm origin

# Check which current directories you want to keep
# (using gui)

# Optionally check if there are "deleted" directories that you want to remove
$files = $( git log --diff-filter=D --summary | ?{ $_.Contains('delete') } | %{ $_ -replace '^ delete mode \d+ (.*)$', '$1' } )
# Directories
$files | %{ $_ -replace '^(.*/)[^/]+$', '$1' } | Select-Object -Unique    
# Top-level directories
$files | %{ $_ -replace '^([^/]+/).*$', '$1' } | Select-Object -Unique

# Remove directory(s) that are NOT the directory you want to keep [A]
git filter-branch --index-filter 'git rm --cached --ignore-unmatch -r DirectoryToRemove Directory\ With\ Spaces\ To\ Remove' --prune-empty -- --all

# git keeps a 'backup reference' of the originals. [B]  These can be removed by:
git for-each-ref --format="%(refname)" refs/original/ | %{ git update-ref -d $_ }
# (needed before re-running git filter-branch unless you use '-f' to overwrite backup)

# Steps [A] and [B] can be repeated

# Optionally move directories to top-level
git filter-branch --tree-filter 'test -d Directory && mv Directory/* . || echo Nothing_To_Do' -- --all
# (removal of the directory 'Directory' is not needed)

# Again, remove backup… (Step [B])

# Drop all dangling data
git gc --prune=now


# STEP 1: Create the BuildScripts repository
# Similar to the procedure above, but do the reverse: 
#   remove BuildScripts instead of all directories that are not the BuildScripts