stg uncommit should never touch the branch head

However, currently, it will set head to top, potentially losing data
(which can always be recovered via the reflog, but still). See
https://gna.org/bugs/index.php?12043. Add a test to demonstrate the
bad behavior. (Bug discovered by Erik Sandberg
<mandolaerik@gmail.com>.)

stg commit, on the other hand, should refuse to run if top != head,
since the committed patches might otherwise be lost. Add a test to
demonstrate that this is the case.

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