Merge Repositories Preserving Both Histories

Scenario:

You have two separate Git repositories at /home/interview/repo-a (5 commits) and /home/interview/repo-b (4 commits) developed independently. Create a new monorepo at /home/interview/monorepo that combines both repositories into separate subdirectories using subtree (project-a/ and project-b/) while preserving the full commit history from both repositories.

Example:

# Before (two separate repositories)
$ cd /home/interview/repo-a && git log --oneline | wc -l
5
$ cd /home/interview/repo-b && git log --oneline | wc -l
4

# After (combined monorepo with both histories)
$ cd /home/interview/monorepo && ls -la
project-a/  project-b/  .git/
$ git log --all --oneline | wc -l
12

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