pretty: initialize new cmt_fmt_map to 0

pretty: initialize new cmt_fmt_map to 0

Without this change, is_alias is likely to happen to be nonzero,
resulting in "fatal: invalid --pretty format" when the fake alias
cannot be resolved.

Use memset instead of initializing the members one by one to make it
easier to expand the struct in the future if needed.

t4205 (log --pretty) does not pass for me without this fix.

Cc: Will Palmer <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>