tests: fix `filtertraceback.py` to handle contiguous "File" lines

It looks like it assumed each `"  File"` line would be followed by the code at
the referenced line, but that's not true of a few things in `test-hook.t`.  That
and the fact that there are an odd number of them in that test caused the
`self.loader.exec_module(module)` line below to leak through in non-chg tests,
when the filter is applied in `test-hook.t`:

     Traceback (most recent call last):
      File "C:\Users\Matt\hg\mercurial\hook.py", line 62, in pythonhook
        obj = __import__(pycompat.sysstr(modname))
      File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
      File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
      File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
      File "C:\Users\Matt\hg\hgdemandimport\demandimportpy3.py", line 52, in exec_module
        self.loader.exec_module(module)
      File "$TESTTMP\b\syntaxerror.py", line 1
        (foo

tests/filtertraceback.py

@@ -37,8 +37,13 for line in sys.stdin:
             continue
 
     elif state == 'file':
-        # Ignore lines after "  File "
-        state = 'tb'
+        # Ignore one line after "  File ", but sometimes "File" lines are
+        # contiguous:
+        #      File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
+        #      File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
+        #      File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
+        if not line.startswith('  File '):
+            state = 'tb'
         continue
 
     print(line, end='')