Patrick Steinhardt [off-list ref] writes:

A common operation when editing the commit history is to drop a specific
commit from the history entirely, but this operation is not currently
covered by git-history(1).

A couple of noteworthy bits:

  - This is the first git-history(1) command that will ultimately result
    in changes to both the index and the working tree. We thus have to
    add logic to merge resulting changes into those.

  - It is still not possible to replay merge commits, so this limitation
    is inherited for the new "drop" command.

  - For now we refuse to drop root commits. While we _can_ indeed drop
    root commits in the general case, there are edge cases where the
    resulting history would become completely empty. This is thus left
    to a subsequent patch series.

Other than that, most of the logic is rather straight-forward as we can
continue to build on the preexisting logic in git-history(1) for most of
the part.

Signed-off-by: Patrick Steinhardt <redacted>
---
...
+static int update_worktree(struct repository *repo,
+			   const struct commit *old_head,
+			   const struct commit *new_head,
+			   bool dry_run)
+{
+...
+
+out:
+	clear_unpack_trees_porcelain(&opts);
+	rollback_lock_file(&lock);
+	release_index(&index);
+	free(desc_buf[0]);
+	free(desc_buf[1]);
+	return ret;
+}

The function looks very familiar---anybody who wants to perform
"checkout <other-commit>" needs to do exactly the above.  It is a
bit surprising and disappointing that this topic needs to *invent*
its own helper function and carry it as a file-scope static.

+	if (head_moves && update_worktree(repo, old_head, new_head, false) < 0) {
+		ret = error(_("failed to update working tree; "
+			      "run `git checkout HEAD` to sync"));
+		goto out;
+	}

This is minor, but unlike in documentation pages written in AsciiDoc, we do
not do backticks for literals in our error messages, I think.