Fix upstream_commitish_search API

Fix upstream_commitish_search API

upstream_commitish_search used to return a commit.  If it succeeded
The caller could find the tag in $tried[-1].  Both callers relied on
this unpleasant and error-prone API.

In
   0bb8e2a87e3c8b5be0fce5c2491b292e9273056e
   Dgit::upstream_commitish_search: fail if more than one tag exists

the algorithm was changed to keep looking, so it can reject ambiguous
situations.  The result is that $tried[-1] is entirely wrong in the
success case.  (This is spotted by the tagupl-baredebian test.)

It would have been possible to fix this by making
upstream_commitish_search synthesise a suitable return value for
putting in $tried, but that is absurd.

Instead give this function a sensible calling convention.  It now
returns a list of the tag name (for messages etc.) and the
commitish (for use).  Change both call sites.

CC: Sean Whitton <spwhitton@spwhitton.name>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>