msubprojects: Checkout if the branch is tracking upstream

A rebase of the current branch is the wrong thing to do if the
revision (branch) specified in the wrap changed, which is the case in
the majority of cases, such as when switching from one release branch
to another.

mesonbuild/msubprojects.py

@@ -287,6 +287,19 @@ class Runner:
             success = self.git_rebase(revision)
         return success
 
+    def git_branch_has_upstream(self, urls: set) -> bool:
+        cmd = ['rev-parse', '--abbrev-ref', '--symbolic-full-name', '@{upstream}']
+        ret, upstream = quiet_git(cmd, self.repo_dir)
+        if not ret:
+            return False
+        try:
+            remote = upstream.split('/', maxsplit=1)[0]
+        except IndexError:
+            return False
+        cmd = ['remote', 'get-url', remote]
+        ret, remote_url = quiet_git(cmd, self.repo_dir)
+        return remote_url.strip() in urls
+
     def update_git(self) -> bool:
         options = T.cast('UpdateArguments', self.options)
         if not os.path.exists(os.path.join(self.repo_dir, '.git')):
@@ -378,10 +391,14 @@ class Runner:
                 success = self.git_rebase(revision)
         else:
             # We are in another branch, either the user created their own branch and
-            # we should rebase it, or revision changed in the wrap file and we need
-            # to checkout the new branch.
+            # we should rebase it, or revision changed in the wrap file (we
+            # know this when the current branch has an upstream) and we need to
+            # checkout the new branch.
             if options.reset:
                 success = self.git_checkout_and_reset(revision)
+            else:
+                if self.git_branch_has_upstream({url, push_url}):
+                    success = self.git_checkout_and_rebase(revision)
                 else:
                     success = self.git_rebase(revision)
         if success: