clone|fetch|--mirror: add convenience $INBOX_DIR/Makefile

Since the beginning of time, I've been dropping Makefiles
in $INBOX_DIR (and above hiearchies) to organize groups
of commands.

make(1) is widely available in various flavors and a familiar
tool for our target audience.  It is easy to run in the right
directory, typically has built-in shell completion, and doesn't
silently ignore errors by default like Bourne shell.

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