[SPARK-28616][INFRA] Improve merge-spark-pr script to warn WIP PRs and strip trailing dots

## What changes were proposed in this pull request?

This PR aims to improve the `merge-spark-pr` script in the following two ways.
1. `[WIP]` is useful when we show that a PR is not ready for merge. Apache Spark allows merging `WIP` PRs. However, sometime, we accidentally forgot to clean up the title for the completed PRs. We had better warn once more during merging stage and get a confirmation from the committers.
2. We have two kinds of PR titles in terms of the ending period. This PR aims to remove the trailing `dot` since the shorter is the better in the commit title. Also, the PR titles without the trailing `dot` is dominant in the Apache Spark commit logs.
```
$ git log --oneline | grep '[.]$' | wc -l
    4090
$ git log --oneline | grep '[^.]$' | wc -l
   20747
```

## How was this patch tested?

Manual.
```
$ dev/merge_spark_pr.py
git rev-parse --abbrev-ref HEAD
Which pull request would you like to merge? (e.g. 34): 25157

The PR title has `[WIP]`:
[WIP][SPARK-28396][SQL] Add PathCatalog for data source V2
Continue? (y/n):
```

```
$ dev/merge_spark_pr.py
git rev-parse --abbrev-ref HEAD
Which pull request would you like to merge? (e.g. 34): 25304
I've re-written the title as follows to match the standard format:
Original: [SPARK-28570][CORE][SHUFFLE] Make UnsafeShuffleWriter use the new API.
Modified: [SPARK-28570][CORE][SHUFFLE] Make UnsafeShuffleWriter use the new API
Would you like to use the modified title? (y/n):
```

Closes #25356 from dongjoon-hyun/SPARK-28616.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: dongjoon-hyun

dev/merge_spark_pr.py

@@ -473,8 +473,13 @@ def main():
 
     url = pr["url"]
 
+    # Warn if the PR is WIP
+    if "[WIP]" in pr["title"]:
+        msg = "The PR title has `[WIP]`:\n%s\nContinue?" % pr["title"]
+        continue_maybe(msg)
+
     # Decide whether to use the modified title or not
-    modified_title = standardize_jira_ref(pr["title"])
+    modified_title = standardize_jira_ref(pr["title"]).rstrip(".")
     if modified_title != pr["title"]:
         print("I've re-written the title as follows to match the standard format:")
         print("Original: %s" % pr["title"])