[RFC] Code reorgnization

[Duy Nguyen <pclouds@gmail.com>]

Git's top directory is crowded and I think it's agreed that moving
test-* to t/helper is a good move. I just wanted to check if we could
take this opportunity (after v2.8.0) to move some other files too. I
propose the following new subdirs

lib
---
This contains files that are about data structures or algorithms. Very
general purpose. This directory includes

argv-array.[ch] base85.c column.[ch] delta.h diff-delta.c hashmap.[ch]
hex.c khash.h kwset.[ch] levenshtein.[ch] mergesort.[ch] patch-delta.c
prio-queue.[ch] sha1-array.[ch] sha1-lookup.[ch] strbuf.[ch]
string-list.[ch] url.[ch] urlmatch.[ch] utf8.[ch] varint.[ch]
versioncmp.c wildmatch.[ch]

odb
---
The grouping of object database files is to easily make connections
between them. Unlike, for example, diff-related files which either
start with "diff" or has that word in the file name to make
connections.

alloc.c blob.[ch] bulk-checkin.[ch] commit-slab.h commit.[ch]
object.[ch] pack.h pack-revindex.[ch] replace_object.c sha1_file.c
streaming.[ch] tag.[ch] tree.[ch]

index
-----
For the same reason of odb subdir. This directory contains

cache-tree.[ch] name-hash.c preload-index.c read-cache.c
split-index.[ch] unpack-trees.[ch]

sys (or maybe util or support)
------------------------------
These are still general purpose but is usually system-related. They
are still far away from git's core logic. I want to separate them to
make it easier to spot "important" files at top dir.

abspath.c color.[ch] copy.c csum-file.[ch] ctype.c date.c editor.c
exec_cmd.[ch] gettext.[ch] gettext.h gpg-interface.[ch] ident.c
lockfile.[ch] mailinfo.[ch] mailmap.[ch] pager.c parse-options-cb.c
parse-options.[ch] pathspec.[ch] pkt-line.[ch] progress.[ch]
prompt.[ch] quote.[ch] run-command.[ch] sideband.[ch] sigchain.[ch]
symlinks.c tar.h tempfile.[ch] thread-utils.[ch] trace.[ch]
unix-socket.[ch] usage.c userdiff.[ch] wrapper.c write_or_die.c zlib.c

Good? Bad? Ugly?
--
Duy