git-backport-diff: Kill git-log after match found

d0c8cbd has indeed made finding a match faster, but the downside
is that the git-log process continues to run in the background even when
we no longer consume its output.  This is a problem particularly for
large patch series, where git-backport-diff may thus spawn hundreds of
subprocesses.

We don't need the git-log process after we found a match, so make it a
real job instead of an anonymous subprocess, which allows us to
terminate it after we have found a match.

Reported-by: Thomas Huth <[email protected]>
Fixes: d0c8cbd

git-backport-diff

@@ -226,20 +226,34 @@ compare_git()
  do
  let cnt=$cnt+1;
  subj=${hashsubj:40}
+
+ # We need to direct the git-log output through a FIFO so we
+ # get a git-log job that we can kill once we have found our
+ # match
+ git_fifo=$(mktemp -u --tmpdir 'backport-diff.XXXXXX')
+ mkfifo "$git_fifo"
+
  downhash=${hashsubj:0:40}
  # A little bit hackish, but find the match by looking at upstream
  # subject lines, and using the newest one. Not all backports contain
  # the phrase "cherry-pick", so we can't really try and find the
  # upstream hash from that...
  uphash=""
+ git log $upstream --pretty=tformat:"%H%s" --fixed-strings --grep="${subj}" >"$git_fifo" &
+ job_id=$(jobs -l | grep "\<$!\>" | sed -e 's/[^\[]*\[\([^\]]*\)\].*/\1/')
  while read uphashsubj
  do
  if [[ "${uphashsubj:40}" == "$subj" ]]
  then
  uphash=${uphashsubj:0:40}
  break
  fi
- done < <(git log $upstream --pretty=tformat:"%H%s" --fixed-strings --grep="${subj}")
+ done <"$git_fifo"
+
+ # Ignore errors
+ kill %$job_id &>/dev/null || true
+ wait &> /dev/null
+ rm -f "$git_fifo"
 
  if [[ -n "$uphash" ]]
  then