Add line break to 'git commit -m' from the command line

Certainly, how it's done depends on your shell. In Bash, you can use single quotes around the message and can just leave the quote open, which will make Bash prompt for another line, until you close the quote. Like this:

git commit -m 'Message

goes
here'

Alternatively, you can use a "here document" (also known as heredoc):

git commit -F- <<EOF
Message

goes
here
EOF

If you just want, say, a head line and a content line, you can use:

git commit -m "My head line" -m "My content line."

Note that this creates separate paragraphs - not lines. So there will be a blank line between each two -m lines, e.g.:

My head line

My content line.