Rework get_main_worktree_path

Author: PawelLipski

git_machete/git_operations.py

@@ -635,15 +635,19 @@ def get_main_worktree_path(self) -> str:
             # If worktrees aren't supported, just return the current root dir
             return self.get_root_dir()
 
+        # We can't rely on `git rev-parse --git-common-dir`
+        # since in some earlier supported versions of git
+        # this path is apparently printed in a faulty way when dealing with worktrees.
+        # Let's parse the output of of `git worktree list` instead.
         result = self._popen_git("worktree", "list", "--porcelain")
 
         # The first worktree in the list is always the main worktree
-        for line in result.stdout.strip().splitlines():
-            if line.startswith('worktree '):  # pragma: no branch
-                return line[len('worktree '):]
-
-        # Fallback: if no worktrees found, return current root dir
-        return self.get_root_dir()  # pragma: no cover
+        first_entry = result.stdout.splitlines()[0]
+        if first_entry.startswith('worktree '):
+            return first_entry[len('worktree '):]
+        else:
+            raise UnexpectedMacheteException("Could not obtain the main worktree path from "
+                                             f"`git worktree list --porcelain` output (first line: `{first_entry}`)")
 
     def checkout(self, branch: LocalBranchShortName) -> None:
         self._run_git("checkout", "--quiet", branch, "--", flush_caches=True)