Merge branch 'ds/midx-normalize-pathname-before-comparison'

The path taken by "git multi-pack-index" command from the end user
was compared with path internally prepared by the tool withut first
normalizing, which lead to duplicated paths not being noticed,
which has been corrected.

* ds/midx-normalize-pathname-before-comparison:
  cache: use const char * for get_object_directory()
  multi-pack-index: use --object-dir real path
  midx: use real paths in lookup_multi_pack_index()

diff --cc cache.h
Simple merge

diff --cc environment.c
Simple merge

diff --cc midx.c
Simple merge