Merge branch 'jc/fix-clone-single-starting-at-a-tag' into maint

"git clone -b brefs/tags/bar" would have mistakenly thought we were
following a single tag, even though it was a name of the branch,
because it incorrectly used strstr().

* jc/fix-clone-single-starting-at-a-tag:
  builtin/clone.c: detect a clone starting at a tag correctly