This is pretty interesting and shows you shouldn't try to pull any sort of stunts if you're not a crypto expert. I've actually wondered before whether md5 + sha1 would result in something stronger than those two used individually. Now I know.
By the way, this may be rather obvious, but concatenating hash algorithms is a terrible idea for passwords. A password cracker could easily pick the less secure algorithm to crack, and ignore the other hash.
Note that git doesn't concern itself with reversing a hash function. The commit contents are part of a repository, there is no value in guessing the commit contents basing on its hash. Here, the hash function choice is purely about collision resistance.
But yeah, don't do weird things with hashes. Cryptography is hard. Don't invent memecrypto: https://twitter.com/sciresm/status/912082817412063233, it's not going to increase the security. Use a single algorithm if you can. Don't transform the output of a hash function in any way.
The linked article doesnt contradict the original post. Linked article says strength of 2 hash algos (of this type) is only as strong as the strongest and not the sum of their strengths. But original poster only needed the combined hash to be as strong as the sha256 for his/her purpose.
