RebaseCommand: Remove unnecessary fall through comments

RebaseCommand: Remove unnecessary fall through comments

It's easier to read a list of cases

	case A:
	case B:
	case C:
		... do something ...;
		break;

without intervening "// fall through" lines separating the cases.

This also makes it clearer that JGit uses "$FALL-THROUGH$", not "fall
through", to suppress fallthrough warnings in switch statements.
Eclipse does not pay attention to the second comment style.

Change-Id: I5279a727aee2868854d06bfcaac8cb8186b4299e
Signed-off-by: Jonathan Nieder <jrn@google.com>