When "git log" shows the log message indented by 4-spaces, the
remainder of a line after a HT does not align in the way the author
originally intended.  The command now expands tabs by default in
such a case, and allows the users to override it with a new option,
'--no-expand-tabs'.

* lt/pretty-expand-tabs:
  pretty: test --expand-tabs
  pretty: allow tweaking tabwidth in --expand-tabs
  pretty: enable --expand-tabs by default for selected pretty formats
  pretty: expand tabs in indented logs to make things line up properly