TL;DR
You can revert a patch with:
$ git apply -R <patch>
You can generate a patch either by one of the following:
This will generate a patch from a diff
$ git diff --patch > 0001-some-modifications.patch
If you want to generate a patch for just the HEAD commit:
$ git show --patch HEAD^ > 0001-some-modifications.patch
You can generate a patch for the previous 3 commits from HEAD:
$ git show --patch HEAD~3 > 0001-some-modifications.patch
You can apply the patch by:
$ git apply -- 0001-some-modifications.patch
You can revert a patch with:
$ git apply -R <patch>
When you generate a patch it is just a diff with metadata; files, line numbers adds/removes; something along the following:
commit 9dad147cbf16befecdef2e812c1249499bdef5ac
Author: My Name <email@example.org>
Date: Mon Dec 21 20:46:01 2015 +0000
Example commit message.
diff --git a/src/example.md b/src/example.md
new file mode 100644
index 0000000..ab73512
--- /dev/null
+++ b/src/example.md
@@ -0,0 +1,3 @@
+# Example document
+
+ Hello World
So when you use git apply you’re essentially applying the edits as per to the tree.
When you then run git apply -R git will simply do the opposite to the patch.