update-microsoft-git: Windows implementation

On Windows, we have the 'git update-git-for-windows' command. It is
poorly named within the microsoft/git fork, because the script has been
updated to look at the GitHub releases of microsoft/git, not
git-for-windows/git.

Still, it handles all the complicated details about downloading,
verifying, and running the installer.

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

builtin/update-microsoft-git.c

@@ -2,11 +2,28 @@
 #include "repository.h"
 #include "parse-options.h"
 #include "run-command.h"
+#include "strvec.h"
 
+#if defined(GIT_WINDOWS_NATIVE)
+/*
+ * On Windows, run 'git update-git-for-windows' which
+ * is installed by the installer, based on the script
+ * in git-for-windows/build-extra.
+ */
 static int platform_specific_upgrade(void)
 {
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	strvec_push(&cp.args, "git-update-git-for-windows");
+	return run_command(&cp);
+}
+#else
+static int platform_specific_upgrade(void)
+{
+	error(_("update-microsoft-git is not supported on this platform"));
 	return 1;
 }
+#endif
 
 static const char builtin_update_microsoft_git_usage[] =
 	N_("git update-microsoft-git");