preserve windows error numbers as well

src/common.h

@@ -74,6 +74,30 @@ void giterr_set(int error_class, const char *string, ...);
 int giterr_set_regex(const regex_t *regex, int error_code);
 
 /**
+ * Gets the system error code for this thread.
+ */
+GIT_INLINE(int) giterr_system_last(void)
+{
+#ifdef GIT_WIN32
+	return GetLastError();
+#else
+	return errno;
+#endif
+}
+
+/**
+ * Sets the system error code for this thread.
+ */
+GIT_INLINE(void) giterr_system_set(int code)
+{
+#ifdef GIT_WIN32
+	SetLastError(code);
+#else
+	errno = code;
+#endif
+}
+
+/**
  * Check a versioned structure for validity
  */
 GIT_INLINE(int) giterr__check_version(const void *structure, unsigned int expected_max, const char *name)

src/fileops.c

@@ -343,11 +343,11 @@ int git_futils_mkdir(
 
 		/* make directory */
 		if (p_mkdir(make_path.ptr, mode) < 0) {
-			int tmp_errno = errno;
+			int tmp_errno = giterr_system_last();
 
 			/* ignore error if directory already exists */
 			if (p_stat(make_path.ptr, &st) < 0 || !S_ISDIR(st.st_mode)) {
-				errno = tmp_errno;
+				giterr_system_set(tmp_errno);
 				giterr_set(GITERR_OS, "Failed to make directory '%s'", make_path.ptr);
 				goto done;
 			}