Auto merge of #371 - metajack:git-fetch-with-retry, r=larsbergstrom
Auto merge of #371 - metajack:git-fetch-with-retry, r=larsbergstrom

Switch to fresh method for the buildbot Git step

This switches from method=clobber to method=fresh, which means instead
of a completely full new clone, we will run git clean -fxxd to reset the
tree and use git fetch to synchronize the repository. This also adds a
single retry on failure for the git fetch step. Failure of the git clean
step will result in a full reclone.

This should alleviate CI failures related to git clone errors, and also
make the step slightly faster and quite a bit lighter on the network load.

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rwaweber/saltfs,servo/saltfs,UK992/saltfs,UK992/saltfs,rwaweber/saltfs,ALikhachev/saltfs,servo/saltfs,ALikhachev/saltfs,ALikhachev/saltfs,UK992/saltfs,rwaweber/saltfs