add -e: do not show difference in a submodule that is merely dirty

add -e: do not show difference in a submodule that is merely dirty

When the HEAD of the submodule matches what is recorded in the index of
the superproject, and it has local changes or untracked files, the patch
offered by "git add -e" for editing shows a diff like this:

    diff --git a/submodule b/submodule
    <header>
    -deadbeef...
    +deadbeef...-dirty

Because applying such a patch has no effect to the index, this is a
useless noise.  Generate the patch with IGNORE_DIRTY_SUBMODULES flag to
prevent such a change from getting reported.

This patch also loses the "-dirty" suffix from the output when the HEAD of
the submodule is different from what is in the index of the superproject.
As such dirtiness expressed by the suffix does not affect the result of
the patch application at all, there is no information lost if we remove
it. The user could still run "git status" before "git add -e" if s/he
cares about the dirtiness.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>