Add new supported variables in commit templates

Proposal

Current supported variables for commit templates are described here...

${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