(git): Add script to diff a single file with all existing branches.

Created with help of ChatGPT.

bin/git-diff-file-for-branches

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# File to compare
+FILE="$1"
+
+if [[ -z "$FILE" ]]; then
+    echo "Usage: $0 <file>"
+    exit 1
+fi
+
+CURRENT_BRANCH=$(git rev-parse --abbrev-ref HEAD)
+
+echo current: $CURRENT_BRANCH
+
+for BRANCH in $(git for-each-ref --format='%(refname:short)' refs/heads/); do
+    if [[ "$BRANCH" == "$CURRENT_BRANCH" ]]; then
+        continue
+    fi
+
+    echo "=== Diff vs $BRANCH ==="
+
+    git diff "$BRANCH" -- "$FILE"
+    echo
+done