Merge branch 'pw/p4-symlinked-root' into pu

Expecting a reroll.
$gmane/217641

* pw/p4-symlinked-root:
  git p4: avoid expanding client paths in chdir
  git p4 test: should honor symlink in p4 client root
  git p4 test: make sure P4CONFIG relative path works
Merge branch 'kb/p4merge' into pu

Expecting a reroll.

* kb/p4merge:
  git-merge-one-file: revise merge error reporting
  mergetools/p4merge: create a base if none available
  mergetools/p4merge: swap LOCAL and REMOTE
Merge branch 'jk/empty-archive' into pu

"git archive" reports a failure when asked to create an archive out
of an empty tree.  It would be more intuitive to give an empty
archive back in such a case.

Will merge to 'next'.

* jk/empty-archive:
  archive: handle commits with an empty tree
  test-lib: factor out $GIT_UNZIP setup