Question

From the Git Book:

“if the current branch has not diverged from the other–so every commit present in the current branch is already contained in the other–then git just performs a “fast forward”

I’m trying to reproduce this scenario, but this doesn’t produce any fast forwards:

$ git init
Initialized empty Git repository in /work/fun/git_experiments/.git/
$ echo initial > readme && git add readme && git commit -a -m Created
[master (root-commit) 74495b9] Created
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 readme
$ git branch b1
$ echo modified > readme && git commit -a -m "Modified"
[master d40d5fb] Modified
 1 files changed, 1 insertions(+), 1 deletions(-)
$ git checkout b1
Switched to branch 'b1'
$ echo modified > readme && git commit -a -m "Modified"
[    b1 46fd337] Modified
 1 files changed, 1 insertions(+), 1 deletions(-)
$ git merge master
Merge made by recursive.
$ 

Answer 1

Because although the content of the two commits are the same, they are not actually the same commit (N.B.: one is commit ID d40d5fb and the other is 46fd337). Thus, branch b1 contains a commit not in master, so not all of the commits in b1 are a descendant of master.