[PATCH v8] Detection of directory renames

Changes since v7:

* turned debug traces off but allow them to be easily turned on
* corrected example in gitdiffcore.txt (Junio)
* unidiff output changes:
  * don't use "diff --git" to avoid confusing git-apply, use
    --git-detect-bulk-moves instead (Junio, 2008)
  * use "bulk move with similarity index NNN%"
  * don't use trailing asterisk, useless there
* added missing &&'s in testsuite
* improved short help messages
* documented the unidiff output format and --hide-bulk-move-details
* splitted raw output format, under discussion, into its own patch
* improved commit messages, including current rationale for using "/*"
* listed bulk-removal/copy/add as possible future related works
* renamed test script for consistency

I'll experiment next with bulk-removal detection, with bulk-move rerolled on top of it.