git status: not "commit --dry-run" anymore

git status: not "commit --dry-run" anymore

This removes tentative "git stat" and make it take over "git status".

There are some tests that expect "git status" to exit with non-zero status
when there is something staged.  Some tests expect "git status path..." to
show the status for a partial commit.

For these, replace "git status" with "git commit --dry-run".  For the
ones that do not attempt a dry-run of a partial commit that check the
output from the command, check the output from "git status" as well, as
they should be identical.

Signed-off-by: Junio C Hamano <gitster@pobox.com>