'git commit -m "$str"' when $str was already terminated with a LF
now avoids adding an extra LF to the message.
* bc/commit-complete-lines-given-via-m-option:
Documentation/git-commit.txt: rework the --cleanup section
git-commit: only append a newline to -m mesg if necessary
t7502: demonstrate breakage with a commit message with trailing newlines
t/t7502: compare entire commit message with what was expected