macos environment: ignore homebrew shim

Ignore Homebrew shim to ensure the user's installed version of Git is
called during `brew uninstall` (instead of the shim). Note that this
involves explicitly passing the HOMEBREW_PREFIX environment variable,
since the `sudo -u `/usr/bin/logname` "$GCMBIN" unconfigure` command
does not inherit it.

Author: ldennington

src/osx/Installer.Mac/uninstall.sh

@@ -12,7 +12,7 @@ fi
 
 # Unconfigure (as the current user)
 echo "Unconfiguring credential helper..."
-sudo -u `/usr/bin/logname` "$GCMBIN" unconfigure
+sudo -u `/usr/bin/logname` -E "$GCMBIN" unconfigure
 
 # Remove symlink
 if [ -L /usr/local/bin/git-credential-manager-core ]

src/shared/Core/CommandContext.cs

@@ -107,7 +107,7 @@ public CommandContext(string appPath)
                 FileSystem        = new MacOSFileSystem();
                 SessionManager    = new MacOSSessionManager();
                 SystemPrompts     = new MacOSSystemPrompts();
-                Environment       = new PosixEnvironment(FileSystem);
+                Environment       = new MacOSEnvironment(FileSystem);
                 Terminal          = new MacOSTerminal(Trace);
                 string gitPath    = GetGitPath(Environment, FileSystem, Trace);
                 Git               = new GitProcess(

src/shared/Core/Interop/MacOS/MacOSEnvironment.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Threading;
+using GitCredentialManager.Interop.Posix;
+
+namespace GitCredentialManager.Interop.MacOS
+{
+    public class MacOSEnvironment : PosixEnvironment
+    {
+        private ICollection<string> _pathsToIgnore;
+
+        public MacOSEnvironment(IFileSystem fileSystem)
+            : base(fileSystem) { }
+
+        internal MacOSEnvironment(IFileSystem fileSystem, IReadOnlyDictionary<string, string> variables)
+            : base(fileSystem)
+        {
+            EnsureArgument.NotNull(variables, nameof(variables));
+            Variables = variables;
+        }
+
+        public override bool TryLocateExecutable(string program, out string path)
+        {
+            if (_pathsToIgnore is null)
+            {
+                _pathsToIgnore = new List<string>();
+                if (Variables.TryGetValue("HOMEBREW_PREFIX", out string homebrewPrefix))
+                {
+                    string homebrewGit = Path.Combine(homebrewPrefix, "Homebrew/Library/Homebrew/shims/shared/git");
+                    _pathsToIgnore.Add(homebrewGit);
+                }
+            }
+            return TryLocateExecutable(program, _pathsToIgnore, out path);
+        }
+    }
+}