Add tldr page for diff

common/diff.md

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+# diff
+
+> Compare files and directories
+
+- Compare files
+
+`diff {{file1}} {{file2}}`
+
+- Compare files, ignoring white spaces
+
+`diff -w {{file1}} {{file2}}`
+
+- Compare files, showing differences side by side
+
+`diff -y {{file1}} {{file2}}`
+
+- Compare directories recursively
+
+`diff -r {{directory1}} {{directory2}}`
+
+- Compare directories, only showing the names of files that differ 
+
+`diff -rq {{directory1}} {{directory2}}`