pretty: enable --expand-tabs by default for selected pretty formats

pretty: enable --expand-tabs by default for selected pretty formats

"git log --pretty={medium,full,fuller}" and "git log" by default
prepend 4 spaces to the log message, so it makes sense to enable
the new "expand-tabs" facility by default for these formats.
Add --no-expand-tabs option to override the new default.

The change alone breaks a test in t4201 that runs "git shortlog"
on the output from "git log", and expects that the output from
"git log" does not do such a tab expansion.  Adjust the test to
explicitly disable expand-tabs with --no-expand-tabs.

Signed-off-by: Junio C Hamano <gitster@pobox.com>