Vim quicktip #5: gq{motion} wraps lines to fit the configured textwidth (:set textwidth). If textwidth is not set, #vim will use the screen width (with a maximum of 79).
Use gqq to format the current line, or gq in visual select mode to format all selected lines.
This is especially useful to make sure the lines in your commit messages aren’t longer than 72 characters. vim-fugitive automatically sets the textwidth option to 72 when writing a commit message with :Gcommit.