Initial implementation of 'git-crypt status'

'git-crypt status' tells you which files are and aren't encrypted and
detects other problems with your git-crypt setup.

'git-crypt status -f' can be used to re-stage files that were incorrectly
staged unencrypted.

The UI needs work, and it needs to also output the overall repository
status (such as, is git-crypt even configured yet?), but this is a
good start.

git-crypt.cpp

@@ -159,6 +159,9 @@ try {
 	if (std::strcmp(command, "refresh") == 0) {
 		return refresh(argc, argv);
 	}
+	if (std::strcmp(command, "status") == 0) {
+		return status(argc, argv);
+	}
 	// Plumbing commands (executed by git, not by user):
 	if (std::strcmp(command, "clean") == 0) {
 		return clean(argc, argv);