This attempts to show useful differences between two repos (probably your fork and the upstream original)

git-compare-upstream.rb

#!/usr/bin/env ruby

# This attempts to show useful differences between an upstream repo and your local copy.
# The motivation is that I want to be able to see differences between my forks of other github repos.

unless upstream_repo = ARGV.shift
  puts "Usage: #{File.basename(__FILE__)} upstream-repo"
  exit 1
end

`git status`
unless $?.success?
  puts "Error. The 'git status' command failed.  I'm guessing this isn't a git repo.  Byeeee."
  exit 1
end

remotes = `git remote -v`
remotes = remotes.split("\n").collect { |remote_and_url| remote_and_url.split("\t") }
upstream_remotes = remotes.select { |remote, url| remote == 'upstream' }
if upstream_remotes.any?
  if upstream_remotes.detect { |remote, url| url =~ /^#{upstream_repo} / }
    warn "Warn. You have a remote named upstream that points at the same URL you've just specified.  This'll probably be fine.  Probably."
  else
    puts "Error. You already have a remote named upstream that has a different URL to the one specified.  To avoid any Bad Things happening I'm going to abort.  Byeeee."
    exit 1
  end
else
  `git remote add upstream #{upstream_repo}`
end

`git fetch upstream --quiet`

branches = `git branch -r`
branches = branches.reject { |branch| branch =~ /HEAD/ }.collect { |branch| branch.gsub(/^ */, '').sub("\n", '').split('/') }
origin_branches   = branches.select { |remote, branch| remote == 'origin' }.collect { |remote, branch| branch }
upstream_branches = branches.select { |remote, branch| remote == 'upstream' }.collect { |remote, branch| branch }

branches_only_in_origin   = origin_branches - upstream_branches
branches_only_in_upstream = upstream_branches - origin_branches
branches_in_common        = origin_branches & upstream_branches

branch_diffs = branches_in_common.inject({}) do |hash, branch|
  commits_only_in_upstream = `git log --pretty=oneline origin/#{branch}..upstream/#{branch}`
  commits_only_in_origin   = `git log --pretty=oneline upstream/#{branch}..origin/#{branch}`
  hash[branch] = [commits_only_in_upstream.split("\n").length, commits_only_in_origin.split("\n").length]
  hash
end

# Print the stats
COL_1_LENGTH = 20
COL_2_LENGTH = 10
COL_3_LENGTH = 10

def in_columns(col_1='', col_2='', col_3='')
  puts [col_1.to_s.ljust(COL_1_LENGTH), col_2.to_s.ljust(COL_2_LENGTH), col_3.to_s.ljust(COL_3_LENGTH)].join
end

puts ''
puts '## Branches only in the upstream repo'
branches_only_in_upstream.each do |branch|
  puts "* #{branch}"
end
puts ''

puts '## Branches only in your repo'
puts ''
branches_only_in_origin.each do |branch|
  puts "* #{branch}"
end
puts ''

puts '## Branch differences'
puts ''
in_columns '', 'Commits in'
in_columns 'Branch', 'Upstream', 'Origin'
branch_diffs.each do |branch, (commits_only_in_upstream, commits_only_in_origin)|
  in_columns branch, commits_only_in_upstream, commits_only_in_origin
end
puts ''