Add page for 'tac'.

pages/common/tac.md

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+# tac
+
+> Print and concatenate files in reverse.
+
+- Print the contents of *file1* reversed to the standard output
+
+`tac {{file1}}`
+
+- Concatenate several files reversed into the target file.
+
+`tac {{file1}} {{file2}} > {{target-file}}`