Split-up "unpack_trees()" cleanup series

Ok, here's the patch I sent out earlier, except now it's been split up into a series fo five patches (and has some trivial one-liner cleanups here and there, mainly to shrink the patches a bit)

It's a series of five patches:
- Add 'df_name_compare()' helper function
- Make 'traverse_tree()' use linked structure rather than 'const char *base'
- Add return value to 'traverse_tree()' callback
- Make 'traverse_trees()' traverse conflicting DF entries in parallel
- Move 'unpack_trees()' over to 'traverse_trees()' interface

with a more lines added than removed:

 cache.h        |    1 +
 merge-tree.c   |   58 ++++---
 read-cache.c   |   35 ++++
 tree-walk.c    |   59 ++++++-
 tree-walk.h    |   23 ++-
 unpack-trees.c |  530 ++++++++++++++++++++++++++------------------------------
 6 files changed, 387 insertions(+), 319 deletions(-)

but especially in this new split-up format, it's now more possible to see how the last patch actually simplifies things.