[llvm] Changed Regex to accept /branch: and /cherry-pick: commands (PR #81002)

[Shourya Goel via llvm-commits]

Sh0g0-1758 wrote:

Taken account of these strings : 

```
"/cherry-pick: Hola mundo"
"/branch: owner/repo/branch"
"/cherry-pick Hola mundo"
"/branch owner/repo/branch"
"/branch : owner/repo/branch"
"/cherry-pick : Hola mundo"
```

https://github.com/llvm/llvm-project/pull/81002