rev-list: default to abbreviate merge parent names under --pretty.

rev-list: default to abbreviate merge parent names under --pretty.

When we prettyprint commit log messages, merge parent names were
often very long and there was no way to abbreviate it.

This changes them to be abbreviated by default, and non-default
abbreviations can be specified with --no-abbrev or --abbrev=<n>
options.

Note that this affects only the prettyprinted parent names.  The
output from --show-parents is meant for machine consumption and
is not affected by this flag.