Comparing 2 files using diff

diff -uN app/models/common_test.rb app/models/major_test.rb  | vi -

Git diff changes staged

# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#       modified:   app/views/matriculations/new.html.erb

To see changes,

git diff --cached (pre-1.6)
git diff --staged

View changes made by upstream in Git

# Fetch the changes from origin
git fetch origin

# View difference between local master branch and origin's master branch
git diff master origin/master