I have started to use git branching more heavily in recent months and also make it a point to have small and well documented commits. This sometimes leads to the need to merge specific files from one branch into another (good or bad, it just happens). Git facilitates this with checkout:

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$ git checkout source-branch-name/commit-sha path/tofile/of/interest

If you do not provide the commit sha it will simply checkout the file at the HEAD of the source branch.