pretty: add aliases for pretty formats

pretty: add aliases for pretty formats

previously the only ways to alias a --pretty format within git were
either to set the format as your default format (via the format.pretty
configuration variable), or by using a regular git alias. This left the
definition of more complicated formats to the realm of "builtin or
nothing", with user-defined formats usually being reserved for quick
one-offs.

Here we allow user-defined formats to enjoy more or less the same
benefits of builtins. By defining format.pretty.myalias, "myalias" can
be used in place of whatever would normally come after --pretty=. This
can be a format:, tformat:, raw (ie, defaulting to tformat), or the name
of another builtin or user-defined pretty format.