check_sdk_version compares with master

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utils/check_sdk_version.py checks the difference between HEAD and master. This worked so far only because this script is a couple of months old, and hasn't yet been on a branch that receives pull requests not targetted at master. But, once we release 1.7...

Thus, it needs to somehow guess what branch we're on. And, a commit that changes master → stable-1.7 won't work well as it'd need to be undone on master, and merging/cherry-picking isn't reliable when such changes go back and forth. Then, someone may want to send a pull request towards a topic branch...

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Here's an idea: let's check if the HEAD commit is a merge. If it is, we can compare its parents (there will be two unless someone plays with octopus — which is not used within our workflow). Otherwise, we're building an unmerged branch, and in this case we can compare with the last merge.

In other words:

• check git log --pretty='%P' -1
• if you get 0 or more than 2 words, don't bother checking (a shallow checkout or an octopus merge)
• if you get two words, compare these two commits
• if you get one word, check git log --merges --pretty='%H -1
• if that returns nothing, skip (a shallow checkout with depth>1)
• otherwise, compare that commit with HEAD

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As you can see from the messages triggered by a private branch, this works for me.