clone: respect additional configured fetch refspecs during initial fetch

clone: respect additional configured fetch refspecs during initial fetch

The initial fetch during a clone doesn't transfer refs matching
additional fetch refspecs given on the command line as configuration
variables, e.g. '-c remote.origin.fetch=<refspec>'.  This contradicts
the documentation stating that configuration variables specified via
'git clone -c <key>=<value> ...' "take effect immediately after the
repository is initialized, but before the remote history is fetched"
and the given example specifically mentions "adding additional fetch
refspecs to the origin remote".  Furthermore, one-shot configuration
variables specified via 'git -c <key>=<value> clone ...', though not
written to the newly created repository's config file, live during the
lifetime of the 'clone' command, including the initial fetch.  All
this implies that any fetch refspecs specified this way should already
be taken into account during the initial fetch.

The reason for this is that the initial fetch is not a fully fledged
'git fetch' but a bunch of direct calls into the fetch/transport
machinery with clone's own refs-to-refspec matching logic, which
bypasses parts of 'git fetch' processing configured fetch refspecs.
This logic only considers a single default refspec, potentially
influenced by options like '--single-branch' and '--mirror'.  The
configured refspecs are, however, already read and parsed properly
when clone calls remote.c:remote_get(), but it never looks at the
parsed refspecs in the resulting 'struct remote'.

Modify clone to take the remote's configured fetch refspecs into
account to retrieve all matching refs during the initial fetch.  Note
that we have to explicitly add the default fetch refspec to the
remote's refspecs, because at that point the remote only includes the
fetch refspecs specified on the command line.

Add tests to check that refspecs given both via 'git clone -c ...' and
'git -c ... clone' retrieve all refs matching either the default or
the additional refspecs, and that it works even when the user
specifies an alternative remote name via '--origin=<name>'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>