apply, entry: speak of submodules instead of subprojects

There are only four (with some generous rounding) instances in the
current source code where we speak of "subproject" instead of
"submodule".  They are as follows:

* one error message in git-apply and two in entry.c

* the patch format for submodule changes

The latter was introduced in 0478675 (Expose subprojects as special
files to "git diff" machinery, 2007-04-15), apparently before the
terminology was settled.  We can of course not change the patch
format.

Let's at least change the error messages to consistently call them
"submodule".

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Thomas Rast

builtin/apply.c

@@ -3847,7 +3847,7 @@ static void add_index_file(const char *path, unsigned mode, void *buf, unsigned
 		const char *s = buf;
 
 		if (get_sha1_hex(s + strlen("Subproject commit "), ce->sha1))
-			die(_("corrupt patch for subproject %s"), path);
+			die(_("corrupt patch for submodule %s"), path);
 	} else {
 		if (!cached) {
 			if (lstat(path, &st) < 0)

entry.c

@@ -199,9 +199,9 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
 		break;
 	case S_IFGITLINK:
 		if (to_tempfile)
-			return error("cannot create temporary subproject %s", path);
+			return error("cannot create temporary submodule %s", path);
 		if (mkdir(path, 0777) < 0)
-			return error("cannot create subproject directory %s", path);
+			return error("cannot create submodule directory %s", path);
 		break;
 	default:
 		return error("unknown file mode for %s in index", path);