确定哪个合并提交导致特定提交合并到我的分支中

Question

Given the following history:

master  A-B-C-D-E-F
             \
topic         G-H-I

What git command can I use to determine that C caused G to be merged into master ?

Said another way, ignoring the branch name: "How can I determine which merge commit caused A to have G in its ancestry?"

Answer 1

git merge-base G master gives the common ancestor:

Find common ancestor of two git branches

I think this answers your first formulation, but I may not understand what you mean when you say " C caused G to be merged into master ".

Answer 2

You can walk forward in time from the first commit until you hit a merge commit that can reach the sought-after commit. By making the walk a first-parent walk we ignore all commits happening in side-branches.

Using the reachability test described in How can I tell if one commit is a descendant of another commit? , the following example finds the first merge commit that introduced the commit identified by the contents of the wanted environment variable to the master branch:

for c in $(git rev-list --reverse --first-parent --merges master) ; do
  test $(git merge-base $wanted $c) = $wanted && echo $c && break
done

For Git 1.8.0 and later, the new --is-ancestor option can shorten the command somewhat:

for c in $(git rev-list --reverse --first-parent --merges master) ; do
  git merge-base --is-ancestor $wanted $c && echo $c && break
done