git: do not move detached HEAD to attic if same commit

pym/bob/scm/git.py

@@ -598,7 +598,7 @@ async def switch(self, invoker, oldScm):
 
             curCommit = await invoker.checkOutputCommand(["git", "rev-parse",
                 "HEAD"], cwd=self.__dir)
-            if curCommit != oldCommit:
+            if curCommit != oldCommit and curCommit != self.__commit:
                 invoker.fail("Cannot switch: user moved to different commit: {} vs. {}".format(curCommit, oldCommit))
 
         # Try to checkout new state in old workspace. If something fails the

test/black-box/git-scm-switch/run.sh

@@ -185,3 +185,29 @@ run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c1" -DSCM_BRANCH=foobar root2 -
 ls -la dev/src/root2/1/workspace
 expect_output "hello world" cat dev/src/root2/1/workspace/test.txt
 expect_not_exist dev/src/root2/1/attic
+
+# detached HEAD handling
+cleanup
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c0" root
+
+# move to detached HEAD state
+pushd dev/src/root/1/workspace
+git checkout $d1_c1
+echo canary > canary.txt
+popd
+
+# run update to the same commit. No attic
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c1" root
+expect_not_exist dev/src/root/1/attic
+
+# commit the change and update to different commit -> attic
+pushd dev/src/root/1/workspace
+git config user.email "[email protected]"
+git config user.name test
+git add .
+git commit -m "local commit"
+popd
+
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c2" root
+expect_not_exist dev/src/root/1/workspace/canary
+expect_exist dev/src/root/1/attic