config --show-origin: report paths with forward slashes

On Windows, the backslash is the native directory separator, but all
supported Windows versions also accept the forward slash in most
circumstances.

Our tests expect forward slashes.

Relative paths are generated by Git using forward slashes.

So let's try to be consistent and use forward slashes in the $HOME part
of the paths reported by `git config --show-origin`, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -584,6 +584,9 @@ char *expand_user_path(const char *path)
 			if (!home)
 				goto return_null;
 			strbuf_addstr(&user_path, home);
+#ifdef GIT_WINDOWS_NATIVE
+			convert_slashes(user_path.buf);
+#endif
 		} else {
 			struct passwd *pw = getpw_str(username, username_len);
 			if (!pw)