Merge branch 'pb/subtree-split-and-merge-after-squashing-tag-fix'

A bugfix to "git subtree" in its split and merge features.

* pb/subtree-split-and-merge-after-squashing-tag-fix:
  subtree: fix split after annotated tag was squashed merged
  subtree: fix squash merging after annotated tag was squashed merged
  subtree: process 'git-subtree-split' trailer in separate function
  subtree: use named variables instead of "$@" in cmd_pull
  subtree: define a variable before its first use in 'find_latest_squash'
  subtree: prefix die messages with 'fatal'
  subtree: add 'die_incompatible_opt' function to reduce duplication
  subtree: use 'git rev-parse --verify [--quiet]' for better error messages
  test-lib-functions: mark 'test_commit' variables as 'local'