Git undo a branch merge

git_undo_merge.txt

With git log check which commit is one prior the merge. Then you can reset it using:

git reset --hard commit_sha
There's also another way

git reset --hard HEAD~1
will get you back 1 commit.

Be aware that any modified and uncommitted/unstashed files will be reset to their unmodified state. 
To keep them either stash changes away or see --merge option below.

Another way:

git reset --hard ORIG_HEAD
might yield better results, as it should preserve your changes. 
ORIG_HEAD will point to a commit directly before merge has occurred, so you don't have to hunt for it yourself.

A tips is to use the --merge switch instead of --hard, it doesn't reset files unnecessarily:

--merge

Resets the index and updates the files in the working tree that are different between <commit> and HEAD, 
but keeps those which are different between the index and working tree (i.e. which have changes which have not been added).