[PATCH] Fix duplicate errmsg in ALTER TABLE SPLIT PARTITION

Applying: Message improvements
Using index info to reconstruct a base tree...
M	src/backend/partitioning/partbounds.c
A	src/test/regress/expected/partition_merge.out
A	src/test/regress/expected/partition_split.out
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): src/test/regress/expected/partition_split.out deleted in HEAD and modified in Message improvements. Version Message improvements of src/test/regress/expected/partition_split.out left in tree.
CONFLICT (modify/delete): src/test/regress/expected/partition_merge.out deleted in HEAD and modified in Message improvements. Version Message improvements of src/test/regress/expected/partition_merge.out left in tree.
Auto-merging src/backend/partitioning/partbounds.c
CONFLICT (content): Merge conflict in src/backend/partitioning/partbounds.c
Patch failed at 0001 Message improvements

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"