Fix usage for list and mtree commands

[gportay]

While I was working on completing the bash completion script (see #215), I found out that both list and mtree commands have an incomplete usage.

Both handle a second optional argument PATH, as the stat command does.

Correct me if I am wrong.

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The three commands list, mtree and stats are dispatched to the same function: verb_list().

Thus, they share the same command line interface:

casync list|mtree|stat [ARCHIVE|ARCHIVE_INDEX|DIRECTORY] [PATH]

However, in the end, the output is different.

This commit fixes the help usage and the man page for both commands list and mtree.