git-commit: set GIT_EDITOR=: if editor will not be launched

This is a preparatory patch that provides a simple way for the future
prepare-commit-msg hook to discover if the editor will be launched.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/hooks.txt

@@ -61,6 +61,10 @@ The default 'pre-commit' hook, when enabled, catches introduction
 of lines with trailing whitespaces and aborts the commit when
 such a line is found.
 
+All the `git-commit` hooks are invoked with the environment
+variable `GIT_EDITOR=:` if the command will not bring up an editor
+to modify the commit message.
+
 commit-msg
 ----------
 

builtin-commit.c

@@ -599,6 +599,8 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		use_editor = 0;
 	if (edit_flag)
 		use_editor = 1;
+	if (!use_editor)
+		setenv("GIT_EDITOR", ":", 1);
 
 	if (get_sha1("HEAD", head_sha1))
 		initial_commit = 1;