1. git send-email is included in an individual package, named "git-email":
$ sudo apt-get install git-email

2. Configure the SMTP server info after the installation:
$ git config --global sendemail.smtpserver smtp.gmail.com
$ git config --global sendemail.smtpserverport 587
$ git config --global sendemail.smtpencryption tls
$ git config --global sendemail.smtpuser 你的邮箱

3. Start a new branch to do any dev work:
$ dev_branch=fix-warning-for-string-format
$ git branch $dev_branch
$ git checkout $dev_branch
$ vim source.c
$ git add source.c
$ git commit -s -m "Add format strings for bb_error_msg_and_die"

4. Format a patch and send it out to the receiver:
$ git format-patch -C -n master..$dev_branch
$ git send-email --compose --no-chain-reply-to --suppress-from --to kernel@kernel.org 0001-*.patch