Make "morph show-system-branch" anywhere in a workspace, if possible

There are mainly three situations to deal with:

1. We are outside a workspace and cannot deduce the system branch.

2. We are inside a workspace and inside a branch. We can detect this
   by walking up from the working directory towards the workspace.
   If we find a .morph-system-branch in one of the parent directories,
   we know the branch name. If we don't find one, something is wrong.

3. We are inside a workspace but outside a branch (or partially into
   a branch, e.g. in foo/ where the branch is foo/bar). We can deduce
   the branch if we recurse into subdirectories to find a
   .morph-system-branch directory. Care needs to be taken to not
   recurse infinitely. We may also not recurse if there are multiple
   subdirectories as these could belong to two different branches.

This commit makes "morph show-system-branch" work in all of the above
scenarios. It also adds tests for all of them.

1 files changed, 1 insertions, 0 deletions

diff --git a/tests.branching/show-system-branch-fails-outside-workspace.exit b/tests.branching/show-system-branch-fails-outside-workspace.exit
new file mode 100644
index 00000000..d00491fd
--- /dev/null
+++ b/tests.branching/show-system-branch-fails-outside-workspace.exit
@@ -0,0 +1 @@
+1