Support --amend on initial commit in status output

Support --amend on initial commit in status output

We check the existence of the parent commit to determine whether the status is
requested for an initial commit. Since the parent commit depends on the
presence of the --amend switch do initial commit detection after command line
arguments have been handled.

Signed-off-by: Jürgen Rühle <redacted>
Signed-off-by: Junio C Hamano <redacted>