fix: #8031
Currently when we use tmp directory to run a experiment, we set
`EXEC_HEAD` , `EXEC_BASELINE` and `EXEC_MERGE` heads locally but forget
to remove it. This cause some problem when we use `exp show`
1. Clean `EXEC_HEAD` , `EXEC_BASELINE` and `EXEC_MERGE` after we pushed
them to tmp_dir
2. Add a unit test for it.
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