Introduction of a ce_namelen field

Thanks to the review of Junio, Duy and Thomas here is a reroll of the
patches, where the name length is separated from the flags in the
in-memory format and which includes a little bit of a performance
optimization by using the ce_namelen field instead of strlen() in
a couple of places thanks to the suggestion of Duy.

[PATCH/RFC v2 1/2] Strip namelen out of ce_flags into a ce_namelen
[PATCH/RFC v2 2/2] Replace strlen(ce->name) with ce_namlen

builtin/apply.c        |    3 ++-
builtin/blame.c        |    3 ++-
builtin/checkout.c     |    3 ++-
builtin/update-index.c |    9 ++++++---
cache.h                |   18 ++++++------------
read-cache.c           |   58 +++++++++++++++++++++++++++++++++++-----------------------
tree.c                 |    7 ++++---
unpack-trees.c         |    5 +++--
8 files changed, 60 insertions(+), 46 deletions(-)