Add new supported variables in commit templates
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### Proposal
Current supported variables for commit templates are described [here](https://docs.gitlab.com/ee/user/project/merge_requests/commit_templates.html#supported-variables-in-commit-templates)...
`${all_commits}` contains the message (body + subject) from all commits in the merge request, we would like to request another variable `${all_subjects}` that contains only **the commit's title (subject)**.
If we had two commits as shown below:
```
Merge branch 'foo' into 'bar'
Add feature ABC for project XYZ
```
```
Complete fix requested by John Doe
Adding most recent version 1.2
```
We would like the variable `${all_subjects}` to display only below two titles:
```
Merge branch 'foo' into 'bar'
Complete fix requested by John Doe
```
Furthermore, if possible, it would be awesome if we can create custom variables that show up the information displayed by any git command, e.g. if I want to display the information from this git command: `git log -2 --date=short --pretty=format:"%cd %s (%an) %h"`, the generated information from above's command is:
- date
- title
- author
- sha
If a new custom variable `${all_subjects_custom}` is created, below information is displayed
```
2024-07-29 Merge branch 'foo' into 'bar' (Jane Doe) eac32ca30sda42
2024-07-29 Complete fix requested by John Doe (John Doe) abascdas3
```
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issue