util: use ~ as a suffix for a temp file in the same directory as a source file Tools like Buck have patterns to ignore the creation of files (in the working copy) that match certain patterns: https://github.com/facebook/buck/blob/39278a4f0701c5239eae148968dc1ed4cc8661f7/src/com/facebook/buck/cli/Main.java#L259-L299 When Buck sees a new source file (as reported by Watchman), it has to invalidate a number of caches associated with the directory that contains the file. Using a standard suffix, such as `~`, would make it easier for Buck and others to filter out these types of file creation events. The other uses of `tempfile.mkstemp()` in Hg do not appear to be problematic because they (generally speaking) do not specify the `dir` parameter, so the new file is created in the system-appropriate temp directory, which is outside the working copy. Test Plan: `make tests` Differential Revision: https://phab.mercurial-scm.org/D468