Fix ./mach filter_intermittents in shallow git clones.

`git log --merges` only shows commits with at least two parents.
Presumably, this only works when the parent commits are in the local repository.
In a shallow clone (`--depth 1`), on the HEAD commit is fetched.

This commit removes the `--merges` filter.
This shouldn’t affect the behavior,
since CI always runs with a merge commit as HEAD.

python/servo/testing_commands.py

@@ -543,7 +543,7 @@ class MachCommands(CommandBase):
             reported = set()
 
             proc = subprocess.Popen(
-                ["git", "log", "--merges", "--oneline", "-1"],
+                ["git", "log", "--oneline", "-1"],
                 stdout=subprocess.PIPE)
             (last_merge, _) = proc.communicate()