update-microsoft-git: use brew on macOS

The steps to update the microsoft-git cask are:

1. brew update
2. brew upgrade --cask microsoft-git

This is adapted from the UpgradeVerb within microsoft/scalar. There is
one important simplification: Scalar needed to check 'brew list --cask'
to find out if the 'scalar' cask or the 'scalar-azrepos' cask was
installed (which determined if the 'microsoft-git' cask was a necessary
dependency). We do not need that here, since we are already in the
microsoft-git cask.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

builtin/update-microsoft-git.c

@@ -17,6 +17,38 @@ static int platform_specific_upgrade(void)
 	strvec_push(&cp.args, "git-update-git-for-windows");
 	return run_command(&cp);
 }
+#elif defined(__APPLE__)
+/*
+ * On macOS, we expect the user to have the microsoft-git
+ * cask installed via Homebrew. We check using these
+ * commands:
+ *
+ * 1. 'brew update' to get latest versions.
+ * 2. 'brew upgrade --cask microsoft-git' to get the
+ *    latest version.
+ */
+static int platform_specific_upgrade(void)
+{
+	int res;
+	struct child_process update = CHILD_PROCESS_INIT;
+	struct child_process upgrade = CHILD_PROCESS_INIT;
+
+	printf("Updating Homebrew with 'brew update'\n");
+
+	strvec_pushl(&update.args, "brew", "update", NULL);
+	res = run_command(&update);
+
+	if (res) {
+		error(_("'brew update' failed; is brew installed?"));
+		return 1;
+	}
+
+	printf("Upgrading microsoft-git with 'brew upgrade --cask microsoft-git'\n");
+	strvec_pushl(&upgrade.args, "brew", "upgrade", "--cask", "microsoft-git", NULL);
+	res = run_command(&upgrade);
+
+	return res;
+}
 #else
 static int platform_specific_upgrade(void)
 {