apply, entry: speak of submodules instead of subprojects

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>