Always use the newest commit in a branch for push events

What does this MR do?

Prior to this MR, when creating a new default branch, the push event would reference the oldest commit in the branch, rather than the newest. This is the opposite of what we do when creating any other branch, or when updating any branch, whether it's the default branch or not.

Even worse, when the new default branch contained more than 100 commits we would return the 100th commit, rather than the oldest. This is unambiguously a bug - that behaviour is not useful at all.

Fix the ordering of commits when pulling data from the default branch so we always use the newest one when creating the default branch.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #195667 (closed)