For example, with this topology:
- o---o---o---B
- /
+ o---o---o---B
+ /
---o---1---o---o---o---A
the merge base between 'A' and 'B' is '1'.
When the history involves criss-cross merges, there can be more than one
'best' common ancestor for two commits. For example, with this topology:
- ---1---o---A
- \ /
- X
- / \
- ---2---o---o---B
+ ---1---o---A
+ \ /
+ X
+ / \
+ ---2---o---o---B
both '1' and '2' are merge-bases of A and B. Neither one is better than
the other (both are 'best' merge bases). When the `--all` option is not given,
`origin/master` may have been rewound and rebuilt, leading to a
history of this shape:
- o---B1
- /
+ o---B1
+ /
---o---o---B2--o---o---o---B (origin/master)
- \
- B3
- \
- Derived (topic)
+ \
+ B3
+ \
+ Derived (topic)
where `origin/master` used to point at commits B3, B2, B1 and now it
points at B, and your `topic` branch was started on top of it back