Multi-Commit Selection for Diff Operations

**Is your feature request related to a problem? Please describe:**
When working with git history in lazygit, I often need to analyze or share changes across multiple specific commits. Currently, I need to individually view or copy diffs for each commit separately. There's no way to copy diff of multiple commits.

**Describe the solution you'd like:**
I would like lazygit to support selecting multiple commits in the commits view and then:
1. Copy the combined diff of all selected commits to the clipboard
2. Open all selected commits in the configured difftool with a single command

**Describe alternatives you've considered:**
- Writing shell scripts that extract diffs for specific commit ranges
- Using git on the command line with commands like `git difftool commit1..commit2`, but this requires leaving lazygit