merge-tree --stdin: flush stdout to avoid deadlock

merge-tree --stdin: flush stdout to avoid deadlock

If a process tries to read the output from "git merge-tree --stdin"
before it closes merge-tree's stdin then it deadlocks. This happens
because merge-tree does not flush its output before trying to read
another line of input and means that it is not possible to cherry-pick a
sequence of commits using "git merge-tree --stdin". Fix this by calling
maybe_flush_or_die() before trying to read the next line of
input. Flushing the output after each merge does not seem to affect the
performance, any difference is lost in the noise even after increasing
the number of runs.

$ git rev-list --merges --parents -n100 origin/master |
	sed 's/^[^ ]* //' >/tmp/merges
$ hyperfine -L flush 0,1 --warmup 1 --runs 30 \
	'GIT_FLUSH={flush} ./git merge-tree --stdin </tmp/merges'
Benchmark 1: GIT_FLUSH=0 ./git merge-tree --stdin </tmp/merges
  Time (mean ± σ):     546.6 ms ±  11.7 ms    [User: 503.2 ms, System: 40.9 ms]
  Range (min … max):   535.9 ms … 567.7 ms    30 runs

Benchmark 2: GIT_FLUSH=1 ./git merge-tree --stdin </tmp/merges
  Time (mean ± σ):     546.9 ms ±  12.0 ms    [User: 505.9 ms, System: 38.9 ms]
  Range (min … max):   529.8 ms … 570.0 ms    30 runs

Summary
  'GIT_FLUSH=0 ./git merge-tree --stdin </tmp/merges' ran
    1.00 ± 0.03 times faster than 'GIT_FLUSH=1 ./git merge-tree --stdin </tmp/merges'

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>