Fix hg: "parse error at 0: not a prefix:" (#12373)

Use two hypen argument `--rev=` instead of `-r=`

Co-authored-by: Efflam Lemaillet <elemaillet@logilab.fr>
Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

news/370392cf-52cd-402c-b402-06d2ff398f89.bugfix.rst

@@ -0,0 +1 @@
+Fix mercurial revision "parse error": use ``--rev={ref}`` instead of ``-r={ref}``

src/pip/_internal/vcs/mercurial.py

@@ -31,7 +31,7 @@ class Mercurial(VersionControl):
 
     @staticmethod
     def get_base_rev_args(rev: str) -> List[str]:
-        return [f"-r={rev}"]
+        return [f"--rev={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, [], ["--rev=123"], {}),
         (Subversion, [], ["-r", "123"], {}),
         # Test extra_args.  For this, test using a single VersionControl class.
         (