mingw: handle absolute paths in expand_user_path()

On Windows, an absolute POSIX path needs to be turned into a Windows
one.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

path.c

@@ -11,6 +11,7 @@
 #include "path.h"
 #include "packfile.h"
 #include "object-store.h"
+#include "exec-cmd.h"
 
 static int get_st_mode_bits(const char *path, int *mode)
 {
@@ -714,6 +715,10 @@ char *expand_user_path(const char *path, int real_home)
 
 	if (path == NULL)
 		goto return_null;
+#ifdef __MINGW32__
+	if (path[0] == '/')
+		return system_path(path + 1);
+#endif
 	if (path[0] == '~') {
 		const char *first_slash = strchrnul(path, '/');
 		const char *username = path + 1;