git: Don't use rev-list's --no-commit-header

[abhinav]

This flag was introduced in Git 2.33,
which is newer than the default on many systems.
We don't absolutely need this flag -- just manually skip the header.

[abhinav]

This change is part of the following stack:

• git: Don't use rev-list's --no-commit-header #363 ◀

_{Change managed by git-spice.}