Ensure the full path to git is used on Windows

# Conflicts:
#	poetry/core/vcs/git.py
#	tests/vcs/test_vcs.py

poetry/core/vcs/git.py

@@ -6,6 +6,8 @@
 from typing import Any
 from typing import Optional
 
+from poetry.core.utils._compat import PY36
+from poetry.core.utils._compat import WINDOWS
 from poetry.core.utils._compat import Path
 from poetry.core.utils._compat import decode
 
@@ -154,14 +156,49 @@ def __str__(self):  # type: () -> str
 GitUrl = namedtuple("GitUrl", ["url", "revision"])
 
 
+_executable: Optional[str] = None
+
+
+def executable():
+    global _executable
+
+    if _executable is not None:
+        return _executable
+
+    if WINDOWS and PY36:
+        # Finding git via where.exe
+        where = "%WINDIR%\\System32\\where.exe"
+        paths = decode(
+            subprocess.check_output([where, "git"], shell=True, encoding="oem")
+        ).split("\n")
+        for path in paths:
+            if not path:
+                continue
+
+            path = Path(path.strip())
+            try:
+                path.relative_to(Path.cwd())
+            except ValueError:
+                _executable = str(path)
+
+                break
+    else:
+        _executable = "git"
+
+    if _executable is None:
+        raise RuntimeError("Unable to find a valid git executable")
+
+    return _executable
+
+
 class GitConfig:
     def __init__(self, requires_git_presence=False):  # type: (bool) -> None
         self._config = {}
 
         try:
             config_list = decode(
                 subprocess.check_output(
-                    ["git", "config", "-l"], stderr=subprocess.STDOUT
+                    [executable(), "config", "-l"], stderr=subprocess.STDOUT
                 )
             )
 
@@ -310,7 +347,9 @@ def run(self, *args, **kwargs):  # type: (*Any, **Any) -> str
             ) + args
 
         return decode(
-            subprocess.check_output(["git"] + list(args), stderr=subprocess.STDOUT)
+            subprocess.check_output(
+                [executable()] + list(args), stderr=subprocess.STDOUT
+            )
         ).strip()
 
     def _check_parameter(self, parameter):  # type: (str) -> None

tests/vcs/test_vcs.py

@@ -1,5 +1,9 @@
+import subprocess
+
 import pytest
 
+from poetry.core.utils._compat import PY36
+from poetry.core.utils._compat import WINDOWS
 from poetry.core.utils._compat import Path
 from poetry.core.vcs.git import Git
 from poetry.core.vcs.git import GitError
@@ -276,3 +280,41 @@ def test_git_checkout_raises_error_on_invalid_repository():
 def test_git_rev_parse_raises_error_on_invalid_repository():
     with pytest.raises(GitError):
         Git().rev_parse("-u./payload")
+
+
+@pytest.mark.skipif(
+    not WINDOWS or not PY36,
+    reason="Retrieving the complete path to git is only necessary on Windows, for security reasons",
+)
+def test_ensure_absolute_path_to_git(mocker):
+    def checkout_output(cmd, *args, **kwargs):
+        if Path(cmd[0]).name == "where.exe":
+            return "\n".join(
+                [str(Path.cwd().joinpath("git.exe")), "C:\\Git\\cmd\\git.exe"]
+            )
+
+        return b""
+
+    mock = mocker.patch.object(subprocess, "check_output", side_effect=checkout_output)
+
+    Git().run("config")
+
+    assert mock.call_args_list[-1][0][0] == [
+        "C:\\Git\\cmd\\git.exe",
+        "config",
+    ]
+
+
+@pytest.mark.skipif(
+    not WINDOWS or not PY36,
+    reason="Retrieving the complete path to git is only necessary on Windows, for security reasons",
+)
+def test_ensure_existing_git_executable_is_found(mocker):
+    mock = mocker.patch.object(subprocess, "check_output", return_value=b"")
+
+    Git().run("config")
+
+    cmd = Path(mock.call_args_list[-1][0][0][0])
+
+    assert cmd.is_absolute()
+    assert cmd.name == "git.exe"