How to add multiple lines to git commit messages

The answer may depend on what shell you use to run git. For example with bash (just tested this on windows using the bash shell installed with git):

git commit -m "this is
a multi-line
message"

because quite simply bash will not assume that hitting return ends the command if it's in the middle of a quoted string.

That said, I've only ever used -m for one-line messages; other options just "make more sense" to me if I need a multi-line message.