convert: use the merge action constant
The constant exists, lets use them. Otherwise we cannot make these constant more
powerful.
Differential Revision: https://phab.mercurial-scm.org/D12113
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL