It's not a tree, it's actually a DAG (Directed Acyclic Graph)

But not every directed acyclic graph is a tree.

Example of a tree (and therefore also a DAG):

5
|
4 7
| |
3 6
|/
2
|
1

Convention in this presentation: arrows implicitly point up, just like in a git log, i.e.:

and so on.

Example of a DAG that is not a tree:

7
|\
4 6
| |
3 5
|/
2
|
1

This is not a tree because there are two ways to reach 7:

But we often say "tree" intead of "DAG" in the context of Git because DAG sounds ugly.

Example of a graph that is not a DAG:

6
^
|
3->4
^  |
|  v
2<-5
^
|
1

This one is not acyclic because there is a cycle 2, 3, 4, 5, 2.

 Ancestors