[PATCH/RFC v2 0/16] Introduce index file format version 5

Fist again apologies for those who were not credited in the first
version of this series.

The first version of the series was here: $gmane/202752.

Changes since the last version:

This series now applies to the latest master.

[PATCH/RFC v2 01/16] Modify cache_header to prepare for other index formats
Moved the cache_header structs to read-cache.c and redefined
cache_version_header in test-index-version.c

[PATCH/RFC v2 02/16] Modify read functions to prepare for other index
Make read_index_v2 static.

[PATCH/RFC v2 04/16] Modify write functions to prepare for other index
Make write_index_v2 static.

[PATCH/RFC v2 05/16] t2104: Don't fail for index versions other than [23]
Changed the test so that it converts the index to v2 at the beginning
of the test.

[PATCH/RFC v2 11/16] Read cache-tree in index-v5
Added a comment for cache_tree_convert_v5 that it is a destructive
function for the directory entries.

Added credits and reworded some commit messages.

Cleaned the patches up.