git-compat-util.h: reduce optimization level to 1 on MinGW env.

Git for Windows crashes when clone Japanese multibyte repository.
- Japanese Base Encoding is Shift-JIS.
- It happens Japanese multibyte directory name and too-long directory path
- Linux(ex. Ubuntu 13.04 amd64) can clone normally.
- example repository is here:

git clone https://github.com/wnoguchi/mingw-checkout-crash.git

- The reproduce crash repository contains following file only.
  - following directory and file name is encoded for this commit log.
  - actually file name is decoded.]
  %E6%97%A5%E6%9C%AC%E8%AA%9E%E3%83%87%E3%82%A3%E3%83%AC%E3%82%AF%E3%83%88%E3%83%AA%201-long-long-long-dirname/%E6%97%A5%E6%9C%AC%E8%AA%9E%E3%83%87%E3%82%A3%E3%83%AC%E3%82%AF%E3%83%88%E3%83%AA%202-long-long-long-dirname/%E6%97%A5%E6%9C%AC%E8%AA%9E%E3%83%87%E3%82%A3%E3%83%AC%E3%82%AF%E3%83%88%E3%83%AA%203-long-long-long-dirname/%E6%97%A5%E6%9C%AC%E8%AA%9E%E3%83%87%E3%82%A3%E3%83%AC%E3%82%AF%E3%83%88%E3%83%AA%204-long-long-long-dirname/%E6%97%A5%E6%9C%AC%E8%AA%9E%E3%83%87%E3%82%A3%E3%83%AC%E3%82%AF%E3%83%88%E3%83%AA%205-long-long-long-dirname/%E3%81%AF%E3%81%98%E3%82%81%E3%81%AB%E3%81%8A%E8%AA%AD%E3%81%BF%E3%81%8F%E3%81%A0%E3%81%95%E3%81%84.txt
- only one commit.

This commit reduce gcc optimization level from O2 to O1 when MinGW Windows environment.

Signed-off-by: Wataru Noguchi <wnoguchi.0727@gmail.com>