commit -a -m: allow the top-level tree to become empty again

commit -a -m: allow the top-level tree to become empty again

In 03267e8656c (commit: discard partial cache before (re-)reading it,
2022-11-08), a memory leak was plugged by discarding any partial index
before re-reading it.

The problem with this memory leak fix is that it was based on an
incomplete understanding of the logic introduced in 7168624c353 (Do not
generate full commit log message if it is not going to be used,
2007-11-28).

That logic was introduced to add a shortcut when committing without
editing the commit message interactively. A part of that logic was to
ensure that the index was read into memory:

	if (!active_nr && read_cache() < 0)
		die(...)

Translation to English: If the index has not yet been read, read it, and
if that fails, error out.

That logic was incorrect, though: It used `!active_nr` as an indicator
that the index was not yet read. Usually this is not a problem because
in the vast majority of instances, the index contains at least one
entry.

And it was natural to do it this way because at the time that condition
was introduced, the `index_state` structure had no explicit flag to
indicate that it was initialized: This flag was only introduced in
913e0e99b6a (unpack_trees(): protect the handcrafted in-core index from
read_cache(), 2008-08-23), but that commit did not adjust the code path
where no index file was found and a new, pristine index was initialized.

Now, when the index does not contain any entry (which is quite
common in Git's test suite because it starts quite a many repositories
from scratch), subsequent calls to `do_read_index()` will mistake the
index not to be initialized, and read it again unnecessarily.

This is a problem because after initializing the empty index e.g. the
`cache_tree` in that index could have been initialized before a
subsequent call to `do_read_index()` wants to ensure an initialized
index. And if that subsequent call mistakes the index not to have been
initialized, it would lead to leaked memory.

The correct fix for that memory leak is to adjust the condition so that
it does not mistake `active_nr == 0` to mean that the index has not yet
been read.

Using the `initialized` flag instead, we avoid that mistake, and as a
bonus we can fix a bug at the same time that was introduced by the
memory leak fix: When deleting all tracked files and then asking `git
commit -a -m ...` to commit the result, Git would internally update the
index, then discard and re-read the index undoing the update, and fail
to commit anything.

This fixes https://github.com/git-for-windows/git/issues/4462

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