cmp: add page

pages/common/cmp.md

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+# cmp
+
+> Compare two files.
+
+- Find the byte number and line number of the first difference between the files:
+
+`cmp {{file1}} {{file2}}`
+
+- Find the byte number and differing bytes of every difference:
+
+`cmp -l {{file1}} {{file2}}`