Space out to under next indent point in previous nonblank line.
An indent point is a non-whitespace character following whitespace.
The following line shows the indentation points in this line.
^ ^ ^ ^ ^ ^ ^ ^ ^
If the previous nonblank line has no indent points beyond the
column point starts at, `tab-to-tab-stop' is done instead, unless
this command is invoked with a numeric argument, in which case it
does nothing.
See also `indent-relative-maybe'.