How can I make git commit messages divide into multiple lines?

I find it much easier to save the commit message to a file, and then use the -F option.

Example:

$ cat > /tmp/commit_msg.txt
DE123 bug fix: incorrect fetching of instance details
- fixed this and that
- also did such and such
$ git commit -F /tmp/commit_msg.txt

You could also use an editor to edit the message file before the commit.

You just use the following command:

$ git commit -m "1. what i changed
> 2. blank line
> 3. why i changed"

In your terminal, just hit 'enter' for a new line. The commit message won't end until you add the closing quote. The git log will look like:

$ git log
commit abcde2f660c707br2d20411581c4183170c3p0c2
Author: Alex Pan <[email protected]>
Date:   Tue Apr 28 20:52:44 2015 -0700

    1. what i changed
    2. blank line
    3. why i changed

Use two --message/-m options, first one for the subject and second one for the body.

Excerpt from documentation:

-m <msg>

--message=<msg>

Use the given as the commit message. If multiple -m options are given, their values are concatenated as separate paragraphs.

In your case it does exactly what you want, inserts a blank line between first and second lines.

git commit -m "Subject: what I changed" -m "Body: why I changed it"

This is useful if you want to amend previously added comment.

The multiple-line format you describe is the recommended one with Git (See DISCUSSION in the documentation of git commit). The simplest way to do it is to use git commit without -m, and write your message in your text editor.