status: contextually notify user about an initial commit

status: contextually notify user about an initial commit

The existing message, "Initial commit", makes sense for the commit template
notifying users that it's their initial commit, but is confusing when
merely checking the status of a fresh repository (or orphan branch)
without having any commits yet.

Change the output of "status" to say "No commits yet" when "git
status" is run on a fresh repo (or orphan branch), while retaining the
current "Initial commit" message displayed in the template that's
displayed in the editor when the initial commit is being authored.

Correspondingly change the output of "short status" to "No commits yet
on " when "git status -sb" is run on a fresh repo (or orphan branch).

A few alternatives considered were,

 * Waiting for initial commit
 * Your current branch does not have any commits
 * Current branch waiting for initial commit

The most succint one among the alternatives was chosen.

[with help on tests from Ævar]

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>