How do I determine the source branch of a particular branch?

I have a branch in git and want to figure out from what branch it originally was branched and at what commit.

Github seems to know, since when you do a pull request it usually automatically sets up what branch it should go into, but I can't figure out how to do this manually from the command line.

Let me add a concrete example:

master -- ongoing development
2.2    -- stable maintenance

A feature branch feature was created (at commit B below) and worked on (B', C' & E') and merged against the source branch to pick up C and D

 feature branch:    B'-C'-C--D--E'
                   /     /       
 source branch: A--B--C--D--E-- ...

Now I want to merge feature back into its source, but I am not sure whether it was originally a branch off master or 2.2. In order to merge the feature into the correct source, is there a programmatic way to find out if source branch is master or 2.2?

Solution 1:

Git only tracks "upstream" information in individual repositories and this information is not static nor shared between separate clones of the same repository.

The command to set this relationship from the command line is:

git branch --set-upstream <branch> [<start-point>]

Looking at the output of git-diff might give you a clue:

git diff <mybranch>..master # commits in master that are not in mybranch
git diff <mybranch>..2.2 # commits in 2.2 that are not in mybranch

It is likely that the one with fewer commits listed is the branch point (but that is not guaranteed, obviously.

You can also use gitk, or git log to take a look around:

gitk --all
git log --graph --color --decorate --oneline --all

Solution 2:

git show-branch [--all]
git merge-base

The first will show you branches and give you merge information. The second will help you understand the relationship between two specific branches (where they last diverged).