Diff between branches
git diff branch1..branch2
which is equivalent to
git diff branch1 branch2

Diff of one file path
git diff branch1..branch2 -- myfile.cs

Diff from a common ancestor
git diff [options] commitA...commitB [--] [path...]
This form is to view the changes on the branch containing and up to commitB, starting at a common ancestor of both commit.
git diff A...B is equivalent to
git diff $(git merge-base A B) B

Rebase onto
git rebase --onto this-branch start-from-here this-branch

Delete a remote branch
git push -d remote-name branch-name

Delete a local branch
git branch -d branch-name

Delete a remote tracking branch
git branch -d -r remote-name/branch-name

Change url of remote
git remote set-url origin new-url

Fetch one branch from a remote
git fetch <remote_name> <branch_name>

Show the number of lines changed
git diff --stat
git diff --numstat