RFR: 1854: Add "clean" label for merge PRs that have no merge resolution

[Zhao Song]

In this patch, PR bot would add clean label to Merge-style PRs if one of the following conditions is met.

1. There isn't a merge commit at HEAD. 
To verify this condition, we can simply check the number of parents that the HEAD commit has. If the HEAD commit has one parent, it is not a merge commit. However, if it has two parents, then it is considered a merge commit.

2. The merge commit at HEAD is empty(has no merge resolutions)
To verify this condition, we should be able to tell the difference between an empty merge commit and a non-empty merge commit.

Using `git show <Hash>` would help us differentiate between this two types of commits.

(1) git show output for empty merge commit

commit 9752b806edafbdc210db00f3099404cd8bd11edf
Merge: 982b5ad5d8b 99f8d05da93
Author: Zhao Song <zhao.song at oracle.com>
Date:   Tue Apr 11 14:26:18 2023 -0700

    Merge branch 'feature'



(2) git show output for non-empty merge commit

commit 242a0c07cf3204c662a2f1d55a2ca3e494f955fd (HEAD -> master)
Merge: af35791d093 0ebf3bd5fe1
Author: Zhao Song <zhao.song at oracle.com>
Date:   Tue Apr 11 14:38:05 2023 -0700

    Merge branch 'feature2'

diff --cc test.java
index 7c736ab8f08,09647d12eda..a3c5725e39d
--- a/test.java
+++ b/test.java
@@@ -3,4 -3,4 +3,5 @@@
  aaaaa
  +++++
  11111
 +33333
+ 22222



We could find that the difference between the two is whether the commit message body is empty or not.

Since we only care about commit message body, we can use the `git show --pretty=format:%b <Hash>` command to extract solely the commit message body.

-------------

Commit messages:
 - SKARA-1854

Changes: https://git.openjdk.org/skara/pull/1500/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1500&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1854
  Stats: 175 lines in 5 files changed: 172 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1500.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1500/head:pull/1500

PR: https://git.openjdk.org/skara/pull/1500