Git undo auto merging on a specific file only, not the whole branch

Question

Is there a way I can undo a auto-merge of one file that has been changed instead of the whole branch in Git?

I want to undo this merge:

Auto-merging public/stylesheets/application.css

to the previous version and leave the rest at its current state.

I tried:

git reset HEAD~1 public/stylesheet/application.css

but seems like it's not working.

Answer 1

When you do:

git reset HEAD~1 public/stylesheet/application.css

... that changes the version of your file in the index (i.e. the staged version) to the version in HEAD~1 - your working copy remains the same. The current documentation explains this as:

This form resets the index entries for all <paths> to their state at <commit%gt;. (It does not affect the working tree, nor the current branch.)

If you do:

git checkout HEAD~1 -- public/stylesheet/application.css

... that will change both the working copy and the staged version to the version from HEAD~1. The -- is there for safety, just in case you have file name that can also be understood as a commit.

Now that you're got that change staged, you could amend the merge commit with:

git commit --amend

... which is fair enough as long has you haven't pushed the merge anywhere, since the merge will still just have changes from one branch or the other.

Answer 2

You probably want git checkout commit-ish -- file.