submodule: explain first attempt failure clearly

When cloning with --recurse-submodules a superproject with at least one
submodule with HEAD pointing to an unborn branch, the clone goes
something like this:

	Cloning into 'test'...
	<messages about cloning of superproject>
	Submodule '<name>' (<uri>) registered for path '<submodule path>'
	Cloning into '<submodule path>'...
	fatal: Couldn't find remote ref HEAD
	Unable to fetch in submodule path '<submodule path>'
	<messages about fetching with SHA-1>
	From <uri>
	 * branch            <hash> -> FETCH_HEAD
	Submodule path '<submodule path>': checked out '<hash>'

In other words, first, a fetch is done with no hash arguments (that is,
a fetch of HEAD) resulting in a "Couldn't find remote ref HEAD" error;
then, a fetch is done given a hash, which succeeds.

The fetch given a hash was added in fb43e31f2b ("submodule: try harder
to fetch needed sha1 by direct fetching sha1", 2016-02-24), and the
"Unable to fetch..." message was downgraded from a fatal error to a
notice in e30d833671 ("git-submodule.sh: try harder to fetch a
submodule", 2018-05-16).

This commit improves the notice to be clearer that we are retrying the
fetch, and that the previous messages (in particular, the fatal errors
from fetch) do not necessarily indicate that the whole command fails. In
other words:

 - If the HEAD-fetch succeeds and we then have the commit we want,
   git-submodule prints no explanation.
 - If the HEAD-fetch succeeds and we do not have the commit we want, but
   the hash-fetch succeeds, git-submodule prints no explanation.
 - If the HEAD-fetch succeeds and we do not have the commit we want, but
   the hash-fetch fails, git-submodule prints a fatal error.
 - If the HEAD-fetch fails, fetch prints a fatal error, and
   git-submodule informs the user that it will retry by fetching
   specific commits by hash.
   - If the hash-fetch then succeeds, git-submodule prints no
     explanation (besides the ones already printed).
   - If the HEAD-fetch then fails, git-submodule prints a fatal error.

It could be said that we should just eliminate the HEAD-fetch
altogether, but that changes some behavior (in particular, some refs
that were opportunistically updated would no longer be), so I have left
that alone for now.

There is an analogous situation with the fetching code in fetch_finish()
and surrounding functions. For now, I have added a NEEDSWORK.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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