morph merge: Rework how we interact with git

Upsides:
  - clearer error messages on conflicts (we no longer dump the git
    output)
  - merge base is available during morphology merging
  - we no longer need to go through the complexity of implementing a
    git merge driver

We now manually fetch and then merge, instead of using git pull. This
is not strictly necessary, but it makes it clearer in the code how
FETCH_HEAD is involved in the process.

6 files changed, 9 insertions, 24 deletions

diff --git a/tests.branching/merge-conflict-chunks.stderr b/tests.branching/merge-conflict-chunks.stderr
index 9692d2ec..57aafa5a 100644
--- a/tests.branching/merge-conflict-chunks.stderr
+++ b/tests.branching/merge-conflict-chunks.stderr
@@ -1,6 +1 @@
-ERROR: merge errors were encountered in the following repositories:
-
-	baserock:stratum2-hello
-	baserock:stratum3-hello
-
-Please manually merge the target ref into test/feature in each case in the test/feature system branch, and then repeat the system branch merge.
+ERROR: merge errors were encountered. Please manually merge the target ref into test/feature in the remote system branch in each case, and then repeat the 'morph merge' operation.
diff --git a/tests.branching/merge-conflict-chunks.stdout b/tests.branching/merge-conflict-chunks.stdout
index 75d50c7f..bf6d3828 100644
--- a/tests.branching/merge-conflict-chunks.stdout
+++ b/tests.branching/merge-conflict-chunks.stdout
@@ -1,10 +1,4 @@
-Merge errors encountered in baserock:stratum2-hello, branch test/stable:
-Auto-merging conflict.txt
-CONFLICT (add/add): Merge conflict in conflict.txt
-Automatic merge failed; fix conflicts and then commit the result.
-
-Merge errors encountered in baserock:stratum3-hello, branch test/stable:
-Auto-merging conflict.txt
-CONFLICT (add/add): Merge conflict in conflict.txt
-Automatic merge failed; fix conflicts and then commit the result.
-
+Merge conflicts in baserock:stratum2-hello:
+	conflict.txt
+Merge conflicts in baserock:stratum3-hello:
+	conflict.txt
diff --git a/tests.branching/merge-conflict-stratum.stderr b/tests.branching/merge-conflict-stratum.stderr
index 3a7c1522..57aafa5a 100644
--- a/tests.branching/merge-conflict-stratum.stderr
+++ b/tests.branching/merge-conflict-stratum.stderr
@@ -1 +1 @@
-ERROR: Merging failed in baserock:morphs: please 'git merge' the latest test/stable into test/feature in the 'test/feature' system branch.
+ERROR: merge errors were encountered. Please manually merge the target ref into test/feature in the remote system branch in each case, and then repeat the 'morph merge' operation.
diff --git a/tests.branching/merge-conflict-stratum.stdout b/tests.branching/merge-conflict-stratum.stdout
index 20064406..cce0973c 100644
--- a/tests.branching/merge-conflict-stratum.stdout
+++ b/tests.branching/merge-conflict-stratum.stdout
@@ -1,6 +1,2 @@
-Merge errors encountered in baserock:morphs, branch test/stable:
-Auto-merging hello-system.morph
-Auto-merging hello-stratum.morph
-CONFLICT (content): Merge conflict in hello-stratum.morph
-Automatic merge failed; fix conflicts and then commit the result.
-
+Merge conflicts in baserock:morphs:
+	hello-stratum.morph
diff --git a/tests.branching/merge-with-stratum-renamed.stderr b/tests.branching/merge-with-stratum-renamed.stderr
index ad144f93..ff408036 100644
--- a/tests.branching/merge-with-stratum-renamed.stderr
+++ b/tests.branching/merge-with-stratum-renamed.stderr
@@ -1 +1 @@
-ERROR: stratum goodbye-stratum was added within this branch and subsequently edited. This is not yet supported: refusing to merge.
+ERROR: goodbye-stratum.morph was not found in TMP/workspace/master/baserock:morphs at ref 804a475402fb6ded64a2749c7adf5b33c7c8ce1e
diff --git a/tests.branching/merge-with-stratum-renamed.stdout b/tests.branching/merge-with-stratum-renamed.stdout
new file mode 100644
index 00000000..e69de29b
--- /dev/null
+++ b/tests.branching/merge-with-stratum-renamed.stdout