This will happen if the given good and bad revision are not direct descendants of each other.

Let’s assume a repository like this (using exemplary names for the commits):

* dffa2 good-commit
* b38f4 a2
* cc19f a1
| * d1f17 bad-commit
| * fbd1f b2
| * f66cc b1
|/
* 09f66 merge-base-commit

What “merge base” means

As the message uses the term “merge base”, it might be helpful to understand that term to understand the message. A “merge base” of two or more commits is the latest commit which is a parent of all of those commits.

Therefore if those commits would be merged, all changes between the “merge base” and those commits will be merged together. Every commit which is a parent of “merge base” is not relevant to the merge, it already is a parent of all involved commits.

Understanding the bisect

The described message will happen in a case like this:

$ git bisect start
$ git bisect good good-commit
$ git bisect bad bad-commit
Bisecting: a merge base must be tested
[09f66] merge-base-commit

What bisecting does is to find the commit which introduced a problem (leading to a bad state), which in this case could lead to a problem:

Bug was not introduced between good-commit and bad-commit

Assume that the error existed in merge-base-commit. In this case it will not be possible to find the commit that introduced the bug in the difference between good-commit and bad-commit. Instead one of the commits a1, a2 and good-commit solves the problem, which is exactly what will happen if you decide the merge base to be bad:

$ git bisect bad
The merge base merge-base-commit is bad.
This means the bug has been fixed between 09f66 and [dffa2].

Problem was introduced between merge-base-commit and bad-commit

On the other hand if the merge base is good, the problem was introduced in b1, b2 or bad-commit. bisect will then continue between merge-base-commit and bad-commit, picking the commit in the middle between those commits and testing if that one is good:

$ git bisect good
Bisecting: 0 revisions left to test after this (roughly 1 step)
[fbd1f] b2