pg_upgrade: fail early if a tablespace dir already exists for new cluster version

Applying: Avoid failure if tablespace dirs exist and empty..
Using index info to reconstruct a base tree...
M	src/bin/pg_upgrade/check.c
M	src/bin/pg_upgrade/pg_upgrade.c
Falling back to patching base and 3-way merge...
Auto-merging src/bin/pg_upgrade/pg_upgrade.c
Auto-merging src/bin/pg_upgrade/check.c
CONFLICT (content): Merge conflict in src/bin/pg_upgrade/check.c
Patch failed at 0001 Avoid failure if tablespace dirs exist and empty..

error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"