Log and undo external modifications

At the beginning of every StGit command, quickly check if the branch
head recorded in the log is the same as the actual branch head; if
it's not, conclude that some non-StGit tool has modified the stack,
and record a log entry that says so. (Additionally, if the log doesn't
exist yet, create it.)

This introduces the possibility that a log entry specifies a head and
a top that aren't equal. So teach undo, redo, and reset to deal with
that case.

Signed-off-by: Karl Hasselström <kha@treskal.com>