Mercurial: Merging one file between branches in one repo

Nope. Mercurial works on a changeset basis.

But you can do a "dummy merge" where you ignore the incoming changes from one of the branches. Before you commit you could then revert selected files to whatever state you want:

% HGMERGE=internal:local hg merge     # keep my files
% hg revert --rev other-branch a.txt  # update a.txt to other branch
% hg commit -m 'Dummy merge to pick a.txt from other-branch.'

Maybe that will help you a bit.

One fairly clean way of getting the desired result is to do it in two steps: first use graft, then second use histedit.

Say this is the starting point and you need to select some portions of C and D to "merge" after E:

A---B---C---D
      \
       -E

Then you would graft C and D on top of E:

A---B---C---D
      \
       -E--C'--D'

Then use hg histedit to edit C' and D'. During the edit you can make any changes you want, but in this case you would just revert any unwanted files, (or even portions of them).

(Note that histedit edit works by temporarily updating your working folder to match the content of the given changeset as though it were not committed yet. So you can easily revert unwanted files and then hg histedit --continue which will effectively replace the edited changeset.)

So the final result would be:

A---B---C---D
      \
       -E--C''--D''

Where the '' revisions were modified as required.

I would say this approach is more beneficial when you have large changesets that probably should have been multiple smaller commits in the first place; this approach allows you to "disentangle" only the parts that you need. Using this for just a single file would be fine but could be overkill.

If you are using an IDE:

1. Merge the old branch with new branch
2. Go inside the the IDE and remove the unwanted changes
3. Generate the diff file
4. Update and clean the new branch
5. Apply the diff in the new branch