I'd like to understand how it could work. For the simple case with a master branch and one feature branche we could number commits as they are merged into master. What about two branches worked on at the same time, with multiple commits and merged to master? If the numbering has to reflect the timestamps of the commits it should be recomputed at every merge which is probably not desirable. However every branch has its own numbering and I don't know if it's desirable to have two numbers for the same commit.

Would you implement it using tags?