msubprojects: Don't rebase if the branch is tracking upstream

A rebase is the wrong thing to do if the subproject's branch is simply
tracking upstream with no local changes, which is the case in the
majority of cases. A rebase will actually fail in the majority of
cases, such as when switching from one release branch to another.

mesonbuild/msubprojects.py

@@ -287,6 +287,16 @@ def git_checkout_and_rebase(self, revision: str) -> bool:
             success = self.git_rebase(revision)
         return success
 
+    def git_local_branch_is_untouched(self, revision: str, branch: str) -> bool:
+        '''Check whether the local branch is still tracking upstream as-is without local changes'''
+        cmd = ['rev-parse', '--abbrev-ref', '--symbolic-full-name', '@{upstream}']
+        (ret, o) = quiet_git(cmd, self.repo_dir)
+        if not ret:
+            # No upstream branch?
+            return False
+        upstream = o[:-1]
+        return quiet_git(['diff', '--quiet', f'{upstream}..{branch}'], self.repo_dir)[0]
+
     def update_git(self) -> bool:
         options = T.cast('UpdateArguments', self.options)
         if not os.path.exists(os.path.join(self.repo_dir, '.git')):
@@ -383,7 +393,10 @@ def update_git(self) -> bool:
             if options.reset:
                 success = self.git_checkout_and_reset(revision)
             else:
-                success = self.git_rebase(revision)
+                if self.git_local_branch_is_untouched(revision, branch):
+                    success = self.git_checkout_and_rebase(revision)
+                else:
+                    success = self.git_rebase(revision)
         if success:
             self.update_git_done()
         return success