Write to a stack log when stack is modified

Write to a stack log when stack is modified

Create a log branch (called <branchname>.stgit) for each StGit branch,
and write to it whenever the stack is modified.

Commands using the new infrastructure write to the log when they
commit a transaction. Commands using the old infrastructure get a log
entry write written for them when they exit, unless they explicitly
ask for this not to happen.

The only thing you can do with this log at the moment is look at it.

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