filter-repo: add 'pragma: no cover' to intentionally unreachable lines

There are several lines equivalent to BUG() calls in git that are
supposed to be unreachable, and which exist just to make debugging the
fundamental system problem or refactoring of the code slightly easier by
trying to give a more immediate notification of a problem.  If these
error cases are ever hit and happen to be wrong, then the individual
will at worst get a stacktrace and the program will abort...but that
might arguably be even more helpful.  Since there is no harm in avoiding
the work of finding ways to break the system to force these lines to be
covered, simply exclude them from line coverage counting.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -375,7 +375,7 @@ class _GitElement(object):
     method such that the output would match the format produced by
     fast-export.
     """
-    raise SystemExit("Unimplemented function: %s.dump()" % type(self).__name__)
+    raise SystemExit("Unimplemented function: %s.dump()" % type(self).__name__) # pragma: no cover
 
   def __str__(self):
     """
@@ -504,7 +504,7 @@ class FileChanges(_GitElement):
     # For 'M' file changes (modify), expect to have id and mode
     if type_ == 'M':
       if mode is None:
-        raise SystemExit("file mode and idnum needed for %s" % filename)
+        raise SystemExit("file mode and idnum needed for %s" % filename) # pragma: no cover
       self.mode = mode
       self.blob_id = id_
 
@@ -530,7 +530,7 @@ class FileChanges(_GitElement):
     elif self.type == 'D':
       file_.write('D %s\n' % quoted_filename)
     else:
-      raise SystemExit("Unhandled filechange type: %s" % self.type)
+      raise SystemExit("Unhandled filechange type: %s" % self.type) # pragma: no cover
 
 class Commit(_GitElementWithId):
   """
@@ -1022,7 +1022,7 @@ class FastExportFilter(object):
     matches = self._refline_regexes[refname].match(self._currentline)
     if not matches:
       raise SystemExit("Malformed %s line: '%s'" %
-                       (refname, self._currentline))
+                       (refname, self._currentline))  # pragma: no cover
     ref = matches.group(1)
     self._advance_currentline()
     return ref
@@ -1660,7 +1660,7 @@ class FastExportFilter(object):
           m = batch_check_output_re.match(line)
           if not m or m.group(2) != 'tag':
             raise SystemExit("Failed to find new id for {} (old id was {})"
-                             .format(refname, old_hash))
+                             .format(refname, old_hash)) # pragma: no cover
           new_hash = m.group(1)
         f.write('{} {} {}\n'.format(old_hash, new_hash, refname))
     if batch_check_process:
@@ -1857,7 +1857,7 @@ class FilteringOptions(object):
         af(parser, namespace, ':'+dirname, '--path-rename')
       else:
         raise SystemExit("Error: HelperFilter given invalid option_string: {}"
-                         .format(option_string))
+                         .format(option_string)) # pragma: no cover
 
   @staticmethod
   def create_arg_parser():
@@ -2069,7 +2069,7 @@ class FilteringOptions(object):
     output = p.stdout.read()
     if '--combined-all-paths' not in output:
       raise SystemExit("Error: need a version of git whose diff-tree command "
-                       "has the --combined-all-paths option")
+                       "has the --combined-all-paths option") # pragma: no cover
 
   @staticmethod
   def get_replace_text(filename):
@@ -2221,7 +2221,7 @@ class RepoAnalyze(object):
         RepoAnalyze.handle_renames(stats, commit, change_types, filenames)
       else:
         raise SystemExit("Unhandled change type(s): {} (in commit {})"
-                         .format(change_types, commit))
+                         .format(change_types, commit)) # pragma: no cover
 
   @staticmethod
   def gather_data(args):
@@ -2319,7 +2319,7 @@ class RepoAnalyze(object):
     # Close the output, ensure rev-list|diff-tree pipeline completed successfully
     dtp.stdout.close()
     if dtp.wait():
-      raise SystemExit("Error: rev-list|diff-tree pipeline failed; see above.")
+      raise SystemExit("Error: rev-list|diff-tree pipeline failed; see above.") # pragma: no cover
 
     return stats
 
@@ -3032,7 +3032,7 @@ class RepoFilter(object):
       del self._orig_refs[ref]
     p.stdin.close()
     if p.wait():
-      raise SystemExit("git update-ref failed; see above")
+      raise SystemExit("git update-ref failed; see above") # pragma: no cover
 
     # Now remove
     if self._args.debug:
@@ -3089,7 +3089,7 @@ class RepoFilter(object):
 
       # Make sure fast-export completed successfully
       if not self._args.stdin and self._fep.wait():
-        raise SystemExit("Error: fast-export failed; see above.")
+        raise SystemExit("Error: fast-export failed; see above.") # pragma: no cover
 
     # If we're not the manager of self._output, we should avoid post-run cleanup
     if not self._managed_output:
@@ -3098,7 +3098,7 @@ class RepoFilter(object):
     # Close the output and ensure fast-import successfully completes
     self._output.close()
     if not self._args.dry_run and self._fip.wait():
-      raise SystemExit("Error: fast-import failed; see above.")
+      raise SystemExit("Error: fast-import failed; see above.") # pragma: no cover
 
     # Notify user how long it took, before doing a gc and such
     msg = "New history written in {:.2f} seconds; now repacking/cleaning..."
@@ -3130,7 +3130,7 @@ class RepoFilter(object):
                                for x in refs_to_nuke]))
         p.stdin.close()
         if p.wait():
-          raise SystemExit("git update-ref failed; see above")
+          raise SystemExit("git update-ref failed; see above") # pragma: no cover
 
       # Write out data about run
       fef.record_metadata(self.results_tmp_dir(),