[SPARK-25018][INFRA] Use Co-authored-by and Signed-off-by git trailer in merge_spark_pr.py

## What changes were proposed in this pull request?

In [Linux community](https://git.wiki.kernel.org/index.php/CommitMessageConventions), `Co-authored-by` and `Signed-off-by` git trailer have been used for awhile.

Until recently, Github adopted `Co-authored-by` to include the work of co-authors in the profile contributions graph and the repository's statistics. It's a convention for recognizing multiple authors, and can encourage people to collaborate in OSS communities.

Git provides a command line tools to read the metadata to know who commits the code to upstream, but it's not as easy as having `Signed-off-by` as part of the message so developers can find who is the relevant committers who can help with certain part of the codebase easier.

For a single author PR, I purpose to use `Authored-by` and `Signed-off-by`, so the message will look like

```
Authored-by: Author's name <authorexample.com>
Signed-off-by: Committer's name <committerexample.com>
```

For a multi-author PR, I purpose to use `Lead-authored-by:` and `Co-authored-by:` for the lead author and co-authors. The message will look like

```
Lead-authored-by: Lead Author's name <leadauthorexample.com>
Co-authored-by: CoAuthor's name <coauthorexample.com>
Signed-off-by: Committer's name <committerexample.com>
```

It's also useful to include `Reviewed-by:` to give credits to the people who participate on the code reviewing. We can add this in the next iteration.

Closes apache#21991 from dbtsai/script.

Lead-authored-by: DB Tsai <[email protected]>
Co-authored-by: Liang-Chi Hsieh <[email protected]>
Co-authored-by: Brian Lindblom <[email protected]>
Co-authored-by: hyukjinkwon <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>

Author: 4 people

dev/merge_spark_pr.py

@@ -142,6 +142,11 @@ def merge_pr(pr_num, target_ref, title, body, pr_repo_desc):
         distinct_authors[0])
     if primary_author == "":
         primary_author = distinct_authors[0]
+    else:
+        # When primary author is specified manually, de-dup it from author list and
+        # put it at the head of author list.
+        distinct_authors = list(filter(lambda x: x != primary_author, distinct_authors))
+        distinct_authors.insert(0, primary_author)
 
     commits = run_cmd(['git', 'log', 'HEAD..%s' % pr_branch_name,
                       '--pretty=format:%h [%an] %s']).split("\n\n")
@@ -154,20 +159,25 @@ def merge_pr(pr_num, target_ref, title, body, pr_repo_desc):
         # to people every time someone creates a public fork of Spark.
         merge_message_flags += ["-m", body.replace("@", "")]
 
-    authors = "\n".join(["Author: %s" % a for a in distinct_authors])
-
-    merge_message_flags += ["-m", authors]
+    committer_name = run_cmd("git config --get user.name").strip()
+    committer_email = run_cmd("git config --get user.email").strip()
 
     if had_conflicts:
-        committer_name = run_cmd("git config --get user.name").strip()
-        committer_email = run_cmd("git config --get user.email").strip()
         message = "This patch had conflicts when merged, resolved by\nCommitter: %s <%s>" % (
             committer_name, committer_email)
         merge_message_flags += ["-m", message]
 
     # The string "Closes #%s" string is required for GitHub to correctly close the PR
     merge_message_flags += ["-m", "Closes #%s from %s." % (pr_num, pr_repo_desc)]
 
+    authors = "Authored-by:" if len(distinct_authors) == 1 else "Lead-authored-by:"
+    authors += " %s" % (distinct_authors.pop(0))
+    if len(distinct_authors) > 0:
+        authors += "\n" + "\n".join(["Co-authored-by: %s" % a for a in distinct_authors])
+    authors += "\n" + "Signed-off-by: %s <%s>" % (committer_name, committer_email)
+
+    merge_message_flags += ["-m", authors]
+
     run_cmd(['git', 'commit', '--author="%s"' % primary_author] + merge_message_flags)
 
     continue_maybe("Merge complete (local ref %s). Push to %s?" % (