checkout: Fix "initial checkout" detection

Earlier commit 5521883 (checkout: do not lose staged removal, 2008-09-07)
tightened the rule to prevent switching branches from losing local
changes, so that staged removal of paths can be protected, while
attempting to keep a loophole to still allow a special case of switching
out of an un-checked-out state.

However, the loophole was made a bit too tight, and did not allow
switching from one branch (in an un-checked-out state) to check out
another branch.

The change to builtin-checkout.c in this commit loosens it to allow this,
by not insisting the original commit and the new commit to be the same.

It also introduces a new function, is_index_unborn (and an associated
macro, is_cache_unborn), to check if the repository is truly in an
un-checked-out state more reliably, by making sure that $GIT_INDEX_FILE
did not exist when populating the in-core index structure.  A few places
the earlier commit 5521883 added the check for the initial checkout
condition are updated to use this function.

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

builtin-checkout.c

@@ -269,8 +269,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 		}
 
 		/* 2-way merge to the new branch */
-		topts.initial_checkout = (!active_nr &&
-					  (old->commit == new->commit));
+		topts.initial_checkout = is_cache_unborn();
 		topts.update = 1;
 		topts.merge = 1;
 		topts.gently = opts->merge;