[Arcanist] add a local (per working copy) config file

Summary:
This adds the concept of a local (i.e. working copy local) config file
to arc.  This config file has the highest precedence for config
settings that may come from any config file.  The config is stored in
.(git|hg|sv)/arc/config, and is read ahead of the project config by
getConfigFromAnySource().

Test Plan:
#Testing arc set-config and arc get-config

[16:57:04 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19410 $ ./bin/arc get-config
(global) default = https://phabricator.fb.com/conduit

[16:57:12 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19410 $ ./bin/arc set-config foo bar
Set key 'foo' = 'bar' in global config.

[16:57:23 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19411 $ ./bin/arc get-config
(global) default = https://phabricator.fb.com/conduit
(global) foo = bar

[16:57:26 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19411 $ ./bin/arc set-config --local foo baz
Set key 'foo' = 'baz' in local config.

[16:57:35 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19412 $ ./bin/arc get-config
(global) default = https://phabricator.fb.com/conduit
(global) foo = bar
(local) foo = baz

[16:57:39 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19412 $ ./bin/arc set-config foo ''
Deleted key 'foo' from global config (was 'bar').

[16:57:49 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19413 $ ./bin/arc get-config
(global) default = https://phabricator.fb.com/conduit
(global) foo =
(local) foo = baz

[16:57:51 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19413 $ ./bin/arc set-config --local foo ''
Deleted key 'foo' from local config (was 'baz').

[16:58:05 Tue Jun 12 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19414 $ ./bin/arc get-config
(global) default = https://phabricator.fb.com/conduit

#testing getConfigFromAnySource by means of lint

[11:26:57 Wed Jun 13 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19612 $ ./bin/arc set-config lint.engine BogusLintEngine
Set key 'lint.engine' = 'BogusLintEngine' in global config.

[11:30:04 Wed Jun 13 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19613 $ ./bin/arc set-config --local lint.engine DummyLintEngine
Set key 'lint.engine' = 'DummyLintEngine' in local config.

[11:30:19 Wed Jun 13 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19587 $ ./bin/arc lint
Exception
Failed to load symbol 'DummyLintEngine'. If this symbol was recently added or moved, your library map may be out of date. You can rebuild the map by running 'arc liberate'. For more information, see: http://www.phabricator.com/docs/phabricator/article/libphutil_Libraries_User_Guide.html
(Run with --trace for a full exception trace.)

[11:30:25 Wed Jun 13 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19586 $ ./bin/arc set-config --local lint.engine ''
Deleted key 'lint.engine' from local config (was 'DummyLintEngine').

[11:30:34 Wed Jun 13 2012] dschleimer@dev4022.snc6 ~/devtools/arcanist
arcanist local_config 19587 $ ./bin/arc lint
 OKAY  No lint warnings.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1233

Differential Revision: https://secure.phabricator.com/D2739