filemerge: move removal of `.orig` extension on temp file close to context
The place where the `.orig` extension is removed in `_maketempfiles()`
doesn't make it clear that it's the backup path, which is why we have
a comment in the code explaining it. Let's instead move it out of the
function and close to where we get it from `backup.path()`, so that
becomes clear.
Differential Revision: https://phab.mercurial-scm.org/D12190
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
}