Fix error that occurs when a diff swaps a file for a symlink

[AJ-505]

Problem

While using the app, I ran into a strange error:

Uncaught Error: CodeView.addItem: duplicate id "<filename>"
    at e.appendItemsInternal (index-Dm4RBqAJ.js:1540:86005)
    at e.setItems (index-Dm4RBqAJ.js:1540:85611)
    at index-Dm4RBqAJ.js:1540:144009
    at Vc (index-Dm4RBqAJ.js:9:91887)
    at ol (index-Dm4RBqAJ.js:9:96369)
    at xl (index-Dm4RBqAJ.js:9:105219)
    at ol (index-Dm4RBqAJ.js:9:96928)
    at xl (index-Dm4RBqAJ.js:9:105219)
    at ol (index-Dm4RBqAJ.js:9:96356)
    at xl (index-Dm4RBqAJ.js:9:105219) 

Upon investigation, I realised that this behaviour only occurred on a diff that had a file which became a symlink.

Solution

Git handles these cases (files -> symlinks) specially: It produces two chunks with the same name: One with the file deleted, and the other a new file entry with the link (the one-liner pointing to the file).

Example, say we had notes.txt that had one line, and is now symlinked to target.txt. The git diff output should look like this:

diff --git a/notes.txt b/notes.txt
deleted file mode 100644
index ce01362..0000000
--- a/notes.txt
+++ /dev/null
@@ -1 +0,0 @@
-hello
diff --git a/notes.txt b/notes.txt
new file mode 120000
index 0000000..4cbb553
--- /dev/null
+++ b/notes.txt
@@ -0,0 +1 @@
+target.txt

Thus, passing that directly to @pierre/diffs's CodeView threw a duplicate id error. The core fix is to merge the duplicates first before merging. Additionally, add guardrails on symlinks to ensure they do not get followed wrongly, or be commented on to ensure intended behavior persists.

Verification

npm run test

This fix also works for symlinks converted to files as well.