Merge branch 'js/am-3-merge-recursive-direct'

"git am -3" calls "git merge-recursive" when it needs to fall back
to a three-way merge; this call has been turned into an internal
subroutine call instead of spawning a separate subprocess.

* js/am-3-merge-recursive-direct:
merge-recursive: flush output buffer even when erroring out
merge_trees(): ensure that the callers release output buffer
merge-recursive: offer an option to retain the output in 'obuf'
merge-recursive: write the commit title in one go
merge-recursive: flush output buffer before printing error messages
am -3: use merge_recursive() directly again
merge-recursive: switch to returning errors instead of dying
merge-recursive: handle return values indicating errors
merge-recursive: allow write_tree_from_memory() to error out
merge-recursive: avoid returning a wholesale struct
merge_recursive: abort properly upon errors
prepare the builtins for a libified merge_recursive()
merge-recursive: clarify code in was_tracked()
die(_("BUG")): avoid translating bug messages
die("bug"): report bugs consistently
t5520: verify that `pull --rebase` shows the helpful advice when failing

diff --cc builtin/am.c
Simple merge

diff --cc grep.c
Simple merge

diff --cc sequencer.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc transport.c
Simple merge

diff --cc wt-status.c
Simple merge