portable: enable FSCache by default

Enable `core.fscache` in the portable git by default as
[discussed](git-for-windows/git#2467 (comment)).

Author: DanPristupov

portable/release.sh

@@ -112,6 +112,7 @@ git config -f "$SCRIPT_PATH/root/$etc_gitconfig" \
 die "Could not configure Git-Credential-Manager as default"
 test 64 != $BITNESS ||
 git config -f "$SCRIPT_PATH/root/$etc_gitconfig" --unset pack.packSizeLimit
+git config -f "$SCRIPT_PATH/root/$etc_gitconfig" core.fscache true
 
 case "$LIST" in
 */git-credential-helper-selector.exe*)