[BUG] (git commit --patch --message $MESSAGE) disallows hunk editing

Tested against v1.7.12.4

Steps to reproduce:
	A. cd $DIRTY_WORKING_COPY
	B. git commit --patch --message $MESSAGE
	C. Stage this hunk? e
Expected behavior:
	After step C, the hunk opens in the user's editor
Actual behavior:
	After step C, the hunk is selected unedited (as if the user had entered "y")

AFAICT, this occurs because of the following code in
builtin/commit.c:parse_and_validate_options:
	if (... || message.len || ...)
		use_editor = 0;
	...
	if (!use_editor)
		setenv("GIT_EDITOR", ":", 1);
Because --message is specified, GIT_EDITOR is set to ":", which
prevents the user from editing hunks, although the intent is most
likely to just prevent the user from editing the commit message.