path: use forward slashes for XDG config on windows

Windows uses backslashes as directory separators. A previous patch [1]
converted backslashes to forward slashes in the `config --show-origin`
output on Windows.

However, we missed the XDG config path which still uses backslashes. To
reproduce the issue on Windows:
    1. Populate `$XDG_CONFIG_HOME/git/config`. For example,
       [xdg]
           foo = bar

    2. Ensure some other file is being sourced for config, e.g. system.

    3. Run `git config --list --show-origin`. The XDG path contains
       backslashes, while the other paths contain forward slashes:
       file:C:/Program Files/Git/etc/gitconfig         system.foo=bar
       file:"C:\\Users\\delilah/.config/git/config"    xdg.foo=bar
       file:C:/Users/delilah/.gitconfig                home.foo=bar
       file:.git/config                                local.foo=bar

For the same reasons as in [1],
    > 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.

We should be consistent and use forward slashes in the XDG path as well.
There are several places to insert a call to `convert_slashes(path)`. I
chose `cleanup_path()` because the name suggests some form of
normalisation and it is already called in the relevant code paths.

Discard the constness of `cleanup_path()` to modifying the path in
place. It calls `skip_prefix(..., const char **out)`, which sets `**out`
to some point in the original string. Hence, it seems we can safely cast
`char**` to `const char**`` for this purpose. But please correct me if
I'm wrong!

[1]: https://lore.kernel.org/git/8beb1c208e33e1de8f272caa22fb7a0b662ca4cc.1458730457.git.johannes.schindelin@gmx.de/

Suggested-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Delilah Ashley Wu <delilahwu@microsoft.com>

Author: delilahw

path.c

@@ -40,13 +40,17 @@ static struct strbuf *get_pathname(void)
 	return sb;
 }
 
-static const char *cleanup_path(const char *path)
+static char *cleanup_path(char *path)
 {
 	/* Clean it up */
-	if (skip_prefix(path, "./", &path)) {
+	if (skip_prefix(path, "./", (const char **)&path))
 		while (*path == '/')
 			path++;
-	}
+
+#ifdef GIT_WINDOWS_NATIVE
+	convert_slashes(path);
+#endif
+
 	return path;
 }