attr: use home directory for ~ includes

Users can specify an attribute file to be included in the home
directory using `~/filename` syntax. Instead of looking in the
global configuration location (which may be overridden), use the
user's _actual_ home directory. This allows callers to change the
global configuration location separately from the home directory.

Author: ethomson

src/libgit2/attrcache.c

@@ -300,7 +300,7 @@ static int attr_cache__lookup_path(
 
 		/* expand leading ~/ as needed */
 		if (cfgval && cfgval[0] == '~' && cfgval[1] == '/') {
-			if (! (error = git_sysdir_expand_global_file(&buf, &cfgval[2])))
+			if (! (error = git_sysdir_expand_homedir_file(&buf, &cfgval[2])))
 				*out = git_str_detach(&buf);
 		} else if (cfgval) {
 			*out = git__strdup(cfgval);