Merge branch 'jk/ext-diff-with-relative'

"git diff --relative" did not mix well with "git diff --ext-diff",
which has been corrected.

* jk/ext-diff-with-relative:
  diff: drop "name" parameter from prepare_temp_file()
  diff: clean up external-diff argv setup
  diff: use filespec path to set up tempfiles for ext-diff

diff --cc diff.c
Simple merge

diff --cc t/t4045-diff-relative.sh
Simple merge