This abstracts away the details of argument quoting, which differs
between Unix and Windows.
Also replace all uses of the system() library call with exec_command().
Although system() exists on Windows, it executes the command via cmd.exe,
which has ridiculous escaping rules.
Run 'git-crypt add-collab KEYID' to authorize the holder of the given
GPG secret key to access the encrypted files. The secret git-crypt key
will be encrypted with the corresponding GPG public key and stored in the
root of the Git repository under .git-crypt/keys.
After cloning a repo with encrypted files, run 'git-crypt unlock'
(with no arguments) to use a secret key in your GPG keyring to unlock
the repository.
Multiple collaborators are supported, however commands to list the
collaborators ('git-crypt ls-collabs') and to remove a collaborator
('git-crypt rm-collab') are not yet supported.
