How to test a merge without actually merging first

Question

Is there any way of simulating a git merge between two branches, the current working branch and the master, but without making any changes?

I often have

Answer 1

If I want to compare changes on a topic branch to master, I find it easiest and safest to do the following:

git checkout master
git checkout -b trial_merge
git merge topic_branch

After completing the merge, it is easy to see the consolidated change from master

git diff master

When done, simply delete the trial_merge branch

git checkout master
git branch -D trial_merge

This way, the master branch never changes.

Answer 2

Here is the solution that I have found: git merge-tree does merging "in memory" and prints the diff without touching your working directory. You can even test a branch without checking it out.

Get the merge diff

First, do this to make sure your repository knows about all the remote branches:

$ git fetch --all

Now use this bash snippet to see how branch $branch would merge into $master:

$ branch='feature'
$ git merge-tree $(git merge-base $branch master) master $branch

No changes are made to your workdir or index. It's a dry-run merge.

Pick information from the output

The output is a diff. In case the branch has been merged, it will be empty.

To find whether there are conflicts, grep it for <<<:

$ git merge-tree $(git merge-base $branch master) master $branch | fgrep '<<<'

To extract conflict diffs, use sed to extract lines between <<< and >>>:

$ git merge-tree $(git merge-base $branch master) master $branch | \
  sed -ne '/^\+<<</,/^\+>>>/ p'

Features

• The diff will be empty if a branch is already merged
• Use grep/sed to extract conflicts information
• Use origin/feature to test branches you've never worked with
• Can be used to see how 2 branches have diverged

Add it to your favorites

Get the diff of the merge:

git config --global alias.mergediff '!f(){ branch="$1" ; into="$2" ; git merge-tree $(git merge-base "$branch" "$into") "$into" "$branch" ; };f '

Usage:

$ git mergediff <feature-branch> <merge-into>
$ git mergediff feature master

Get merge conflicts:

git config --global alias.mergetest '!f(){ git mergediff $@ | sed -ne "/^+<<</,/^+>>>/ p" ; };f '

Usage:

$ git mergetest <feature-branch> <merge-into>
$ git mergetest feature master