subrepo: invoke "_sanitize()" also after "git merge --ff"
Before this patch, sanitizing ".hg/hgrc" in git subrepo doesn't work,
when the working directory is updated by "git merge --ff".
"_sanitize()" is not invoked after checking target revision out into
the working directory in this case, even though it is invoked
indirectly via "checkout" (or "rawcheckout") in other cases.
This patch invokes "_sanitize()" explicitly also after "git merge
--ff" execution.
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