commit: do not use cryptic "new_index" in end-user facing messages

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>