Merge branch 'jk/robustify-parse-commit'

* jk/robustify-parse-commit:
checkout: do not die when leaving broken detached HEAD
use parse_commit_or_die instead of custom message
use parse_commit_or_die instead of segfaulting
assume parse_commit checks for NULL commit
assume parse_commit checks commit->object.parsed
log_tree_diff: die when we fail to parse a commit

diff --cc builtin/blame.c
Simple merge

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc builtin/fast-export.c
Simple merge

diff --cc builtin/name-rev.c
Simple merge

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

diff --cc commit.c
Simple merge

diff --cc commit.h
Simple merge

diff --cc fetch-pack.c
Simple merge

diff --cc log-tree.c
Simple merge

diff --cc sha1_name.c
Simple merge

diff --cc shallow.c
Simple merge

diff --cc upload-pack.c
Simple merge