restore: Show details when extracted repo refs were found corrupt

Noticed this while changing how pull works and making error there
incidentally with leaving more "refs/" prefix. With the error before
this patch tests show:

        git-backup_test.go:91: git-backup_test.go:204: lab.nexedi.com/kirr/git-backup.cmd_restore: 2 errors:
			- E: extracted /tmp/t-git-backup981909377/1/dir 2 + β/repo with+fragile name %αβγ.git refs corrupt:
			- E: extracted /tmp/t-git-backup981909377/1/dir/hello.git refs corrupt:

with the patch tests report:

        git-backup_test.go:91: git-backup_test.go:204: lab.nexedi.com/kirr/git-backup.cmd_restore: 2 errors:
                        - E: extracted /tmp/t-git-backup981909377/1/dir 2 + β/repo with+fragile name %αβγ.git refs corrupt:

                want:
                cbb6d3f205749888f77fb1a88fbac3b8a0b8000f refs/refs/heads/master

                have:
                cbb6d3f205749888f77fb1a88fbac3b8a0b8000f refs/heads/master
                        - E: extracted /tmp/t-git-backup981909377/1/dir/hello.git refs corrupt:

                want:
                647e137fd3b31939b36889eba854a298ef97b6ff refs/refs/heads/branch2
                feeed96ca75fcf8dcf183008f61dbf72e91ab4de refs/refs/heads/master
                11e67095628aa17b03436850e690faea3006c25d refs/refs/tags/tag-to-blob
                f735011c9fcece41219729a33f7876cd8791f659 refs/refs/tags/tag-to-commit
                7124713e403925bc772cd252b0dec099f3ced9c5 refs/refs/tags/tag-to-tag
                ba899e5639273a6fa4d50d684af8db1ae070351e refs/refs/tags/tag-to-tree
                7a3343f584218e973165d943d7c0af47a52ca477 refs/refs/test/ref-to-blob
                61882eb85774ed4401681d800bb9c638031375e2 refs/refs/test/ref-to-tree

                have:
                647e137fd3b31939b36889eba854a298ef97b6ff refs/heads/branch2
                feeed96ca75fcf8dcf183008f61dbf72e91ab4de refs/heads/master
                11e67095628aa17b03436850e690faea3006c25d refs/tags/tag-to-blob
                f735011c9fcece41219729a33f7876cd8791f659 refs/tags/tag-to-commit
                7124713e403925bc772cd252b0dec099f3ced9c5 refs/tags/tag-to-tag
                ba899e5639273a6fa4d50d684af8db1ae070351e refs/tags/tag-to-tree
                7a3343f584218e973165d943d7c0af47a52ca477 refs/test/ref-to-blob
                61882eb85774ed4401681d800bb9c638031375e2 refs/test/ref-to-tree

Should be good to have this details if something really breaks after restore.

git-backup.go

@@ -913,7 +913,9 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
                     }
                     repo_ref_list := strings.Join(repo_ref_listv, "\n")
                     if x_ref_list != repo_ref_list {
-                        exc.Raisef("E: extracted %s refs corrupt", p.repopath)
+                        // TODO show refs diff, not 2 dumps
+                        exc.Raisef("E: extracted %s refs corrupt:\n\nwant:\n%s\n\nhave:\n%s",
+                            p.repopath, repo_ref_list, x_ref_list)
                     }
 
                     // check connectivity in recreated repository.