Add FetchCommand#setRefSpecs(String...) variant

Add FetchCommand#setRefSpecs(String...) variant

Much of the time the caller can specify a RefSpec succinctly using a
string, and doesn't care about calling setters. Add a convenience method
for this case, and use it where applicable in JGit core.

Change-Id: Ic3fac7fc568eee4759236a5264d2e7e5f9b9716d