htmlreport: support remote GitHub/GitLab links for source files

Motivation:
Cppcheck-htmlreport previously generated local annotated HTML for all source files.
For private or large repositories, generating local HTML is unnecessary and cumbersome.
It consumes additional disk space, increases report generation time, and duplicates
functionality already provided by GitHub/GitLab browseable HTML pages.
This patch allows the cppcheck report itself to be public, while the actual source code
remains protected on GitHub/GitLab using their standard access controls.

Changes:
- Detect --source-dir URLs pointing to GitHub/GitLab.
- Use remote URLs in index.html instead of generating local HTML for those files.
- Line numbers link directly to GitHub/GitLab with proper anchors (#L123).
- Remote links open in a new tab (target="_blank"), preserving local HTML behavior for normal files.

Author: rgetz

htmlreport/cppcheck-htmlreport

@@ -494,9 +494,20 @@ def tr_str(td_th, line, id, cwe, severity, classification, guideline, message, t
     if classification:
         items.extend([classification, guideline])
     if htmlfile:
-        ret += '<%s><a href="%s#line-%d">%d</a></%s>' % (td_th, htmlfile, line, line, td_th)
+        if htmlfile.startswith("http://") or htmlfile.startswith("https://"):
+            # GitHub/GitLab style line anchor
+            href = f"{htmlfile.rstrip('#L1')}#L{line}"
+            # Emit **line number with link**
+            ret += f'<{td_th}><a href="{href}" target="_blank" rel="noopener noreferrer">{line}</a></{td_th}>'
+        else:
+            # local HTML annotated
+            href = f"{htmlfile}#line-{line}"
+            # Emit **line number with link**
+            ret += f'<{td_th}><a href="{href}">{line}</a></{td_th}>'
+
+        # Emit id, cwe, severity, classification, ...
         for item in items:
-            ret += '<%s>%s</%s>' % (td_th, item, td_th)
+            ret += f'<{td_th}>{item}</{td_th}>'
     else:
         items.insert(0,line)
         for item in items:
@@ -705,6 +716,10 @@ def main() -> None:
     if options.source_dir:
         source_dir = options.source_dir
 
+    is_remote = False
+    if source_dir.startswith("http://") or source_dir.startswith("https://"):
+        is_remote = True
+
     add_author_information = []
     if options.add_author_information:
         fields = [x.strip() for x in options.add_author_information.split(',')]
@@ -753,9 +768,14 @@ def main() -> None:
     for error in contentHandler.errors:
         filename = error['file']
         if filename not in files:
-            files[filename] = {
-                'errors': [], 'htmlfile': str(file_no) + '.html'}
-            file_no = file_no + 1
+            if is_remote:
+                # Construct remote URL for GitHub/GitLab
+                # tr_str() will use the actual line number, so we can just start with line 1
+                remote_url = source_dir.rstrip('/') + '/' + filename + '#L1'
+                files[filename] = {'errors': [], 'htmlfile': remote_url}
+            else:
+                files[filename] = {'errors': [], 'htmlfile': str(file_no) + '.html'}
+                file_no += 1
         files[filename]['errors'].append(error)
 
     # Make sure that the report directory is created if it doesn't exist.
@@ -795,20 +815,25 @@ def main() -> None:
         if filename == '':
             continue
 
-        source_filename = os.path.join(source_dir, filename)
-        try:
-            with io.open(source_filename, 'r', encoding=options.source_encoding) as input_file:
-                content = input_file.read()
-        except IOError:
-            if error['id'] != 'unmatchedSuppression':
-                sys.stderr.write("ERROR: Source file '%s' not found.\n" %
+        if is_remote:
+            # Remote source: do NOT generate local annotated HTML files.
+            # The index will still point directly to GitHub/GitLab URLs.
+            continue;
+        else:
+            source_filename = os.path.join(source_dir, filename)
+            try:
+                with io.open(source_filename, 'r', encoding=options.source_encoding) as input_file:
+                    content = input_file.read()
+            except IOError:
+                if error['id'] != 'unmatchedSuppression':
+                    sys.stderr.write("ERROR: Source file '%s' not found.\n" %
+                                     source_filename)
+                continue
+            except UnicodeDecodeError:
+                sys.stderr.write("WARNING: Unicode decode error in '%s'.\n" %
                                  source_filename)
-            continue
-        except UnicodeDecodeError:
-            sys.stderr.write("WARNING: Unicode decode error in '%s'.\n" %
-                             source_filename)
-            decode_errors.append(source_filename[2:])  # "[2:]" gets rid of "./" at beginning
-            continue
+                decode_errors.append(source_filename[2:])  # "[2:]" gets rid of "./" at beginning
+                continue
 
         htmlFormatter = AnnotateCodeFormatter(linenos=True,
                                               style='colorful',