Merge branch 'jk/show-branch-lift-name-len-limit'

"git show-branch" expected there were only very short branch names
in the repository and used a fixed-length buffer to hold them
without checking for overflow.

* jk/show-branch-lift-name-len-limit:
show-branch: use skip_prefix to drop magic numbers
show-branch: store resolved head in heap buffer
show-branch: drop head_len variable

diff --cc builtin/show-branch.c
Simple merge