Fix reload issues for files with multiple registered documents

If a file is removed, the file system watcher can loose its watch
on it (e.g. on Linux, inode-based).
Saving files does so "safely" by writing a temporary file, moving
the original, and moving the new file in its place, which is
triggering the issue above. That is why we need to re-register
the path in the file system watcher.

This broke in the refactoring done
by 05485071b0

Task-number: QTCREATORBUG-18892
Change-Id: I3b216d614b9f82e308da63c07d990e5911193655
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/coreplugin/documentmanager.cpp

@@ -261,21 +261,24 @@ static void addFileInfo(IDocument *document, const QString &filePath,
         const QFileInfo fi(filePath);
         state.modified = fi.lastModified();
         state.permissions = fi.permissions();
-        // Add watcher if we don't have that already
+        // Add state if we don't have already
         if (!d->m_states.contains(filePathKey)) {
             FileState state;
             state.watchedFilePath = filePath;
             d->m_states.insert(filePathKey, state);
-
+        }
+        // Add or update watcher on file path
+        // This is also used to update the watcher in case of saved (==replaced) files or
+        // update link targets, even if there are multiple documents registered for it
+        const QString watchedFilePath = d->m_states.value(filePathKey).watchedFilePath;
         qCDebug(log) << "adding (" << (isLink ? "link" : "full") << ") watch for"
-                         << state.watchedFilePath;
+                     << watchedFilePath;
         QFileSystemWatcher *watcher = 0;
         if (isLink)
             watcher = d->linkWatcher();
         else
             watcher = d->fileWatcher();
-            watcher->addPath(state.watchedFilePath);
-        }
+        watcher->addPath(watchedFilePath);
 
         d->m_states[filePathKey].lastUpdatedState.insert(document, state);
     }