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>
diff --git a/builtin/commit.c b/builtin/commit.c
index 985a044..7abd566 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -446,7 +446,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 		refresh_cache_or_die(refresh_flags);
 		cache_tree_update(&the_index, WRITE_TREE_SILENT);
 		if (write_locked_index(&the_index, &index_lock, 0))
-			die(_("unable to write new_index file"));
+			die(_("unable to write new index file"));
 		commit_style = COMMIT_NORMAL;
 		ret = get_lock_file_path(&index_lock);
 		goto out;
@@ -470,7 +470,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 			cache_tree_update(&the_index, WRITE_TREE_SILENT);
 		if (write_locked_index(&the_index, &index_lock,
 				       COMMIT_LOCK | SKIP_IF_UNCHANGED))
-			die(_("unable to write new_index file"));
+			die(_("unable to write new index file"));
 		commit_style = COMMIT_AS_IS;
 		ret = get_index_file();
 		goto out;
@@ -518,7 +518,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 	refresh_index(&the_index, REFRESH_QUIET, NULL, NULL, NULL);
 	cache_tree_update(&the_index, WRITE_TREE_SILENT);
 	if (write_locked_index(&the_index, &index_lock, 0))
-		die(_("unable to write new_index file"));
+		die(_("unable to write new index file"));
 
 	hold_lock_file_for_update(&false_lock,
 				  git_path("next-index-%"PRIuMAX,
@@ -1852,7 +1852,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	if (commit_index_files())
 		die(_("repository has been updated, but unable to write\n"
-		      "new_index file. Check that disk is not full and quota is\n"
+		      "new index file. Check that disk is not full and quota is\n"
 		      "not exceeded, and then \"git restore --staged :/\" to recover."));
 
 	git_test_write_commit_graph_or_die();