Issue Summary

This occurs for git versions 2.38.0+ where a change was introduced to `git merge` to always stash changes before conducting the merge: https://github.com/git/git/commit/034195ef927bcfc348a69672494f3781e8f5c093

For repos that are very large and have many files (millions), it may take a long time to merge pull requests as now git has to record the current state of the working directory. E.g. for ~8million files `git stash` takes around 100s. 

This issues occurs regardless of setting the property `plugin.bitbucket-git.trymerge.use-mergetree`

Steps to Reproduce

1. Have a large repo with millions of files
2. Set `pullrequest.merge.timeout` to a value that's slightly more than the time it takes to run `git stash`
3. Try merge a PR

Expected Results

git processes are cleaned up even when the merge itself times out

Actual Results

The following git processes are left behind, even on Bitbucket restart:

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
atlbitb+ 17912  1.2  9.6 7740264 6260468 ?     S    04:05   1:19 /usr/libexec/git-core/git stash create
atlbitb+ 18017  100  2.7 1804088 1759696 ?     R    04:06 106:03 /usr/libexec/git-core/git update-index --ignore-skip-worktree-entries -z --add --remove --stdin
atlbitb+ 17869  0.3  0.0      0     0 ?        Z    04:05   0:20 [git] <defunct>

Workaround

Manually remove the leftover git processes, or downgrade git to pre 2.38