Repo 1: started from 1/1/2016 to 1/1/2020.
Repo 2: copied from last commit of repo 1 and continues till now
###Concatenating repo 2 to tail of repo 1:
From repo 2, add remote to repo 1:
git remote add old file://to/repo1/
git fet old
Checkout from old repo 1:
git checkout old/master old
Checkout from master (just to make a backup if you do something wrong):
git checkout master work-in-progress
Now I will replay commits from old repo 1 to current working branch (work-in-progress or wip same as master):
git rebase old -i
After this command, git will open a text editor list all commits of new repo 2 that we want to be tailed to repo 1's commit history.
Actually it will replay commits of current branch wip to the tail of old and place all history to wip.
You should check the list and "drop" unwanted commits (edit "pick" to "drop") such as first commit to repo 2 because this commit just added files to the repo.
Now you would encounter some conflict: CONFLICT (content): Merge conflict in file/that/conflicted.
Resolve conflict by content of newer commits. I did that by using TortoiseGIT on Windows by selecting resolving use the left file.
Continue rebasing:
git rebase --continue
Repeat doing resolving and rebasing till all commits are replayed.