Merge branch 'ps/build-sign-compare'

Last-minute fix for a regression in "git blame --abbrev=<length>"
when insane <length> is specified; we used to correctly cap it to
the hash output length but broke it during the cycle.

* ps/build-sign-compare:
  builtin/blame: fix out-of-bounds write with blank boundary commits
  builtin/blame: fix out-of-bounds read with excessive `--abbrev`