Fix behaviour of commit. It's now a fatal error if the option to --logfile isn't readable. Ditto if both --message and --logfile are specified.

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123