When the user initiates a commit, Magit calls git commit without any
arguments, so Git has to get it from the user. It creates the file
.git/COMMIT_EDITMSG and then opens that file in an editor. Magit
arranges for that editor to be the Emacsclient. Once the user
finishes the editing session, the Emacsclient exits and Git creates the
commit using the file’s content as message.