filemerge: make `_maketempfiles()` more reusable `_maketempfiles()` is very specialized for its current use. I hope to use it also when creating temporary files for input for tools that do partial conflict resolution. That'll be possible if the function is more generic. Instead of passing in two contexts (for "other" and "base") and an optional path (for "local"), let's pass a single list of files to make backups for. Even if we don't end up using for partial conflict resolution, this is still a simplification (but I do have a WIP patch for partial conflict resolution and it is able to benefit from this). Differential Revision: https://phab.mercurial-scm.org/D12193

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

filemerge: use leverage `util.readfile()` in `_maketempfiles()` Differential Revision: https://phab.mercurial-scm.org/D12191

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

filemerge: remove `uselocalpath` argument from `_maketempfiles()` The `localpath` argument is unused if `uselocalpath` is false, so we can use `None` as a sentinel value for the variable instead and remove extra `uselocalpath` argument. That's not much of a win, but it's a small step towards further improvements. Differential Revision: https://phab.mercurial-scm.org/D12189

filemerge: remove an unnecessary join with absolute path The `backup` path is now always absolute, so we don't need to join it with the working copy path. Differential Revision: https://phab.mercurial-scm.org/D12188