RFR: 1854: Add "clean" label for merge PRs that have no merge resolution [v4]

[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.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  fix problems

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1500/files
  - new: https://git.openjdk.org/skara/pull/1500/files/7e1b5455..8cbbfc01

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1500&range=03
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1500&range=02-03

  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 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