This ruby script can mix your Textmate project file into a new one with file ignores from a .gitignore file

gitignore2tmproj.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'plist'
require 'fileutils'

plist = ARGV[0] || Dir.pwd.split('/').last
new_plist = "#{plist}.new.tmproj"
gitignore = ARGV[1] || '.gitignore'

if File.exist?(plist) && File.exist?(gitignore)
  # create a new plist file to preserve the original tmproj
  FileUtils.cp(plist, new_plist)

  properties = Plist::parse_xml(new_plist)
  file_filter = []
  folder_filter = []

  File.read(gitignore).split("\n").each do |exclude|
    # all file extension filters
    if exclude =~ /^\*([\.\w]+)$/
      file_filter << Regexp.escape($1)
    # other file and directory filters
    elsif exclude =~ /(.+)(\/\*)?$/
      folder_filter << Regexp.escape($1)
    end
  end

  # update the filter properties
  document = properties['documents'].shift
  document.merge!(
    'regexFileFilter' => "!(#{file_filter.join('|')})$",
    'regexFolderFilter' => "!.*/(\\.[^/]*|#{folder_filter.join('|')})"
  )
  properties['documents'].unshift(document)
  File.open(new_plist, 'w') do |file|
    file.write(Plist::Emit.dump(properties))
  end

  puts "Successfully merged .gitignore into file/folder filters of #{plist}"
else
  puts "  Error: Could not read #{plist} or .gitignore."
  puts
  puts "  Usage: gitignore2tmproj.rb [your.tmproj] [.gitignore]"
  puts "  Defaults to: gitignore2tmproj.rb working_directory_name.tmproj .gitignore"
end