Add "How to merge multiple git repositories and keep commit history"

git/merge-git-repos-and-keep-commit-history.md

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+# How to merge multiple git repositories and keep commit history
+
+### Goals:
+
+1. Merge several separate repos into one.
+2. Preserve the complete Git commit history.
+
+```
+# Before: two repos
+repo_a
+    └── dir_a
+    └── dir_b
+    └── dir_c
+repo_b
+    └── dir_x
+    └── dir_y
+    └── dir_z
+```
+
+```
+# After: new repo X containing both a and b
+repo_X
+    └── repo_a
+        └── dir_a
+        └── dir_b
+        └── dir_c
+    └── repo_b
+        └── dir_x
+        └── dir_y
+        └── dir_z
+```
+
+### Overview
+
+This is an overview of the basic steps.
+
+1. Re-arrange each repo so they can merge without conflicts
+2. Add each repo as a remote source for the repo you want to merge into
+3. Merge the repos with --allow-unrelated-histories
+
+### Prepare each repo for merging
+
+1. make sure you don't have overlapping directories and/or files
+   if you do, try to rename them or put them in directory with the repo name.
+2. Commit and push the changes of the repo
+
+### Connect the repos
+
+```
+# Add a remote for and fetch the old repo while on repo_X
+git remote add -f repo_a <repo_a URL>
+
+# Merge the files from old_a/master into new/master while in repo_X
+git merge old_a/master
+```
+
+https://gfscott.com/blog/merge-git-repos-and-keep-commit-history/
+https://hacks.mozilla.org/2022/08/merging-two-github-repositories-without-losing-commit-history/