Log Message:
Changes 1.8.2.3:
* "rev-list --stdin" and friends kept bogus pointers into the input
  buffer around as human readable object names.  This was not a
  huge problem but was exposed by a new change that uses these
  names in error output.
* When "git difftool" drove "kdiff3", it mistakenly passed --auto
  option that was meant while resolving merge conflicts.
* "git remote add" command did not diagnose extra command line
  arguments as an error and silently ignored them.
Also contains a handful of trivial code clean-ups, documentation
updates, updates to the test suite, etc.