filemerge: reduce some duplication in `_maketempfiles()`
The two callers of the local `maketempfrompath()` function used the
returned file object in the same way. We can reduce duplication by
moving that code into the function.
Differential Revision: https://phab.mercurial-scm.org/D12192
# See http://EditorConfig.org for the specification
root = true
[*.py]
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[*.{c,h}]
indent_size = 8
indent_style = tab
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[*.t]
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