commit: do not use cryptic "new_index" in end-user facing messages
These error messages say "new_index" as if that spelling has some significance to the end users (e.g. the file "$GIT_DIR/new_index" has some issues), but that is not the case at all. The i18n folks were made to include the word literally in the translated messages, which was not a good idea at all. Spell it "new index", as we are just telling the users that we failed to create a new index file. The term is expected to be translated to the end-users' languages, not left as if it were a literal file name. This dates all the way back to the first re-implemenation of "git commit" command in C (the scripted version did not have such wording in its error messages), in f5bbc322 (Port git commit to C., 2007-11-08). Signed-off-by: Junio C Hamano <gitster@pobox.com>