How to find the last commit on a base branch before a merge (with a different branch) occurred?

Question

I would like to revert a pushed merge into a base branch to the state it was right before the merge. How can I do this?

Answer 1

Assuming that you know the merge commit to be reverted and only need to find the base of the merge, as I understand from your comments:

First there is a visual way which shows the parent commits. This might help to understand and make sure you get the right commit:

git show [commit] (and git log too) will print information about your merge commit in this form:

commit [commit]
Merge: [parent1] [parent2]
Author: [author]
Date: Tue Jul 19 12:00:00 2016

Where parent2 is the merged commit while parent1 is the commit is has been merged to. Thus this example (git rev-parse in this case shows the hash of the given branch):

$ git branch
master
$ git rev-parse master
hasha
$ git rev-parse develop
hashb
$ git merge develop

Will result in a commit like this one:

commit newhash
Merge: hasha hashb
Author: [author]
Date: Tue Jul 19 12:00:00 2016

You can also get the respective commits using the ^ shortcut. master^1 (which is the same as master^) will get you the first parent (hasha in the example above), while master^2 will give you the second parent (hashb). So you can also use these for your revert (in your case you would want to revert to master^1 if your branch were master). You might want to try them out calling git show master^ to watch which commit is found this way.

Answer 2

Besides lucash's answer (which is correct and I've upvoted), if you truly want to identify the merge base commit(s), you can use git merge-base on the parent commits.

For instance, suppose that the merge in question is commit a123456 (after being shortened to 7 characters). As lucash said the two parents are a123456^1 and a123456^2 (assuming there are only two parents—for an octopus merge, where there are three or more parents, just keep adding more parent specifiers, e.g., a123456^3 through a123456^99 or whatever). Note that, as the git merge-base documentation says, you need the --all flag to show all merge bases, in the case where there is more than one merge base.

(If there is more than one merge base, the default recursive merge strategy will merge them to produce a single "virtual merge base", which is not actually contained in the repository. You will need the parameters of the merge in order to repeat this process, if you wish to view the virtual merge base result by creating it as an actual physical merge. While general idea should be pretty obvious, the details are beyond the scope of this answer.)

Note that there is a specific gitrevisions syntax for listing "all parents of a merge": a123456^@ is equivalent to listing a123456^1 a123456^2 ... a123456^99 in the hypothetical case of our 99-parent merge commit a123456. Hence, if the name master currently points to the merge commit you just made, you can simply run:

git merge-base --all master^@

to see the hashes of the merge base(s) of the current merge commit.

---

All that said, if you truly want to revert a merge, you generally just want to use git revert -m <number> <mergecommit>; and if you want to review the two sides that went in, you generally want something like:

git rev-list --left-right --cherry-mark --boundary master^1...master^2

(note the three dots), or the same with rev-list replaced with log or log --oneline or similar (incidentally, note that git log and git rev-list are really almost the same command, just with different output formats). The --boundary option tells Git to include the merge bases themselves (but not their parents), marking them with a leading - (which goes well with the --left-right and --cherry-mark options). If you want to exclude the merge bases, omit the --boundary flag. See the (very long) git rev-list documentation for more details.