Support multi spec files in single package.

Support multi spec files in single package.

fix #438.

* get all spec files in fill_packs_from_git()
* split codes that deal with reversion cache codes into three
    functions read_cache(), write_cache() and clean_cache()
* split exporting codes into gbs_export()
* split codes that query git reversion into query_git_commit_rev()
* add a debug message for my_system call
* comment a debug message of checking dependence, it print out
    every second and overwhelm other useful debug message.

Change-Id: I5b295015818d9953b34386e65c74c1b204ede120