GitDestination/GerritDestination descriptive toString().
GitDestination/GerritDestination descriptive toString().

Removes options and other fields that are not part of the
configuration and shouldn't be in the string representation.

Change-Id: Icc9fd8694b6b81e3a4a3fc8589405ed42d92cc8b

google/copybara,google/copybara,google/copybara