Git. Merging changes from one file to another files WITHIN the same branch

Question

I have two files:

    master/newsletter1/file.html 
    master/newsletter2/file.html

newsletter1/file.html has a lot of new changes that I

Answer 1

I would recommend using a third-party diff tool of your choice, in order to do a manual merge between the two files.
This is not about source-control (ie managing the history of versions for one file).
This is about reporting changes from one files to another (the source-control tool will then pick up the new version for the second file).

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As Jefromi mentions in the comments, and as he explains in "run git merge algorithm on two individual files", git merge-file can actually merge two different files, provided a common ancestor is provided.

git merge-file <current-version> <common-ancestor> <other-version>

In your case, you could try with the common ancestor being identical to the current file.

git merge-file master/newsletter2/file.html master/newsletter1/file.html master/newsletter2/file.html