Re-implement Git version parsing with regex

After packaging drops LegacyVersion, version.parse() will no longer
guarantee to be able to parse Git versions, so we need to invent our own
parser. Since we really only care about the major and minor segments,
the logic is pretty simple.

src/pip/_internal/vcs/git.py

@@ -6,9 +6,6 @@
 import urllib.request
 from typing import List, Optional, Tuple
 
-from pip._vendor.packaging.version import _BaseVersion
-from pip._vendor.packaging.version import parse as parse_version
-
 from pip._internal.exceptions import BadCommand, InstallationError
 from pip._internal.utils.misc import HiddenText, display_path, hide_url
 from pip._internal.utils.subprocess import make_command
@@ -29,6 +26,14 @@
 logger = logging.getLogger(__name__)
 
 
+GIT_VERSION_REGEX = re.compile(
+    r"^git version "  # Prefix.
+    r"(\d+)"  # Major.
+    r"\.(\d+)"  # Dot, minor.
+    r"(?:\.(\d+))?"  # Optional dot, patch.
+    r".*$"  # Suffix, including any pre- and post-release segments we don't care about.
+)
+
 HASH_REGEX = re.compile('^[a-fA-F0-9]{40}$')
 
 # SCP (Secure copy protocol) shorthand. e.g. 'git@example.com:foo/bar.git'
@@ -83,21 +88,14 @@ def is_immutable_rev_checkout(self, url, dest):
         )
         return not is_tag_or_branch
 
-    def get_git_version(self):
-        # type: () -> _BaseVersion
-        VERSION_PFX = 'git version '
+    def get_git_version(self) -> Tuple[int, ...]:
         version = self.run_command(
             ['version'], show_stdout=False, stdout_only=True
         )
-        if version.startswith(VERSION_PFX):
-            version = version[len(VERSION_PFX):].split()[0]
-        else:
-            version = ''
-        # get first 3 positions of the git version because
-        # on windows it is x.y.z.windows.t, and this parses as
-        # LegacyVersion which always smaller than a Version.
-        version = '.'.join(version.split('.')[:3])
-        return parse_version(version)
+        match = GIT_VERSION_REGEX.match(version)
+        if not match:
+            return ()
+        return tuple(int(c) for c in match.groups())
 
     @classmethod
     def get_current_branch(cls, location):
@@ -301,7 +299,7 @@ def switch(self, dest, url, rev_options):
     def update(self, dest, url, rev_options):
         # type: (str, HiddenText, RevOptions) -> None
         # First fetch changes from the default remote
-        if self.get_git_version() >= parse_version('1.9.0'):
+        if self.get_git_version() >= (1, 9):
             # fetch tags in addition to everything else
             self.run_command(['fetch', '-q', '--tags'], cwd=dest)
         else:

tests/unit/test_vcs.py

@@ -4,7 +4,6 @@
 from unittest.mock import patch
 
 import pytest
-from pip._vendor.packaging.version import parse as parse_version
 
 from pip._internal.exceptions import BadCommand, InstallationError
 from pip._internal.utils.misc import hide_url, hide_value
@@ -483,7 +482,7 @@ def test_subversion__get_url_rev_options():
 
 def test_get_git_version():
     git_version = Git().get_git_version()
-    assert git_version >= parse_version('1.0.0')
+    assert git_version >= (1, 0, 0)
 
 
 @pytest.mark.parametrize('use_interactive,is_atty,expected', [