Bugfix: Add host name filter shorthand flag for `stats` command

The default value for `--host` flag was set to 'H' (the shorthand version of
the flag), this caused the lookup for the latest snapshot to fail.

Add shorthand flag `-H` for `--host` (with empty default so if these flags
are not specified the latest snapshot will not filter by host name).

Also add shorthand `-H` for `backup` command.

https://github.com/restic/restic/issues/2040
