build new version of git-backup and go language

As I am not familiar with go language, I was not able to easily build git-backup. Thanks to @tc, we found out that all files are not linked, so we have to do:

$ go build *.go -o git-backup

BTW, we were also curious to know why crypto/sha1 was not used. Is it because of assembly language in original code that may fail in case of custom CPU architectures?

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