Use -r=... instead of -r ... for hg

This ensures that the resulting revision can not be misinterpreted as an
option.

Author: pradyunsg

src/pip/_internal/vcs/mercurial.py

@@ -31,7 +31,7 @@ class Mercurial(VersionControl):
 
     @staticmethod
     def get_base_rev_args(rev: str) -> List[str]:
-        return ["-r", rev]
+        return [f"-r={rev}"]
 
     def fetch_new(
         self, dest: str, url: HiddenText, rev_options: RevOptions, verbosity: int

tests/unit/test_vcs.py

@@ -66,7 +66,7 @@ def test_rev_options_repr() -> None:
         # First check VCS-specific RevOptions behavior.
         (Bazaar, [], ["-r", "123"], {}),
         (Git, ["HEAD"], ["123"], {}),
-        (Mercurial, [], ["-r", "123"], {}),
+        (Mercurial, [], ["-r=123"], {}),
         (Subversion, [], ["-r", "123"], {}),
         # Test extra_args.  For this, test using a single VersionControl class.
         (