Major revamp: new key paradigm, groundwork for GPG support

The active key is now stored in .git/git-crypt/key instead of being
stored outside the repo.  This will facilitate GPG support, where the
user may never interact directly with a key file.  It's also more
convenient, because it means you don't have to keep the key file
around in a fixed location (which can't be moved without breaking
git-crypt).

'git-crypt init' now takes no arguments and is used only when initializing
git-crypt for the very first time.  It generates a brand-new key, so
there's no longer a separate keygen step.

To export the key (for conveyance to another system or to a collaborator),
run 'git-crypt export-key FILENAME'.

To decrypt an existing repo using an exported key, run 'git-crypt unlock
KEYFILE'.  After running unlock, you can delete the key file you passed
to unlock.

Key files now use a new format that supports key versioning (which will
facilitate secure revocation in the future).

I've made these changes as backwards-compatible as possible.  Repos
already configured with git-crypt will continue to work without changes.
However, 'git-crypt unlock' expects a new format key.  You can use
the 'git-crypt migrate-key KEYFILE' command to migrate old keys to the
new format.

Note that old repos won't be able to use the new commands, like
export-key, or the future GPG support.  To migrate an old repo, migrate
its key file and then unlock the repo using the unlock command, as
described above.

While making these changes, I cleaned up the code significantly, adding
better error handling and improving robustness.

Next up: GPG support.

commands.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012 Andrew Ayer
+ * Copyright 2012, 2014 Andrew Ayer
  *
  * This file is part of git-crypt.
  *
@@ -31,12 +31,28 @@
 #ifndef _COMMANDS_H
 #define _COMMANDS_H
 
+#include <string>
 
-void clean (const char* keyfile);
-void smudge (const char* keyfile);
-void diff (const char* keyfile, const char* filename);
-void init (const char* argv0, const char* keyfile);
-void keygen (const char* keyfile);
+struct Error {
+	std::string	message;
+
+	explicit Error (std::string m) : message(m) { }
+};
+
+// Plumbing commands:
+int clean (int argc, char** argv);
+int smudge (int argc, char** argv);
+int diff (int argc, char** argv);
+// Public commands:
+int init (int argc, char** argv);
+int unlock (int argc, char** argv);
+int add_collab (int argc, char** argv);
+int rm_collab (int argc, char** argv);
+int ls_collabs (int argc, char** argv);
+int export_key (int argc, char** argv);
+int keygen (int argc, char** argv);
+int migrate_key (int argc, char** argv);
+int refresh (int argc, char** argv);
 
 #endif