Merge branch 'jt/refspec-dwim-precedence-fix'

"git fetch $there refs/heads/s" ought to fetch the tip of the
branch 's', but when "refs/heads/refs/heads/s", i.e. a branch whose
name is "refs/heads/s" exists at the same time, fetched that one
instead by mistake.  This has been corrected to honor the usual
disambiguation rules for abbreviated refnames.

* jt/refspec-dwim-precedence-fix:
  remote: make refspec follow the same disambiguation rule as local refs

diff --cc refs.c
Simple merge

diff --cc remote.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge