Merge branch 'jk/interpret-branch-name'

"git branch @" created refs/heads/@ as a branch, and in general the
code that handled @{-1} and @{upstream} was a bit too loose in
disambiguating.

* jk/interpret-branch-name:
checkout: restrict @-expansions when finding branch
strbuf_check_ref_format(): expand only local branches
branch: restrict @-expansions when deleting
t3204: test git-branch @-expansion corner cases
interpret_branch_name: allow callers to restrict expansions
strbuf_branchname: add docstring
strbuf_branchname: drop return value
interpret_branch_name: move docstring to header file
interpret_branch_name(): handle auto-namelen for @{-1}

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/merge.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc refs.c
Simple merge

diff --cc strbuf.h
Simple merge