merge-tools: allow marking a mergetool as completely disabled Very often in my life I'm finding that the only configured merge tool present on the system is vimdiff[0], and it's currently impossible (as far as I can tell) short of specifying `ui.merge = `[1] to actually *disable* a merge tool. This allows vimdiff-haters to put: [merge-tools] vimdiff.disable = yes in their ~/.hgrc and never see vimdiff again. I'm stopping short of putting this as a commented out entry in the sample new user hgrc (seen when a user runs `hg config --edit` with no ~/.hgrc) for now, but I might come back and do that later. 0: vimdiff is at an awkward intersection: it's usually installed by the vim package which is often installed as a vi substitute, so it's mere presence doesn't imply me wanting it, unlike (say) kdiff3. 1: There's a related problem I ran into today where specifying `ui.merge = :merge` failed because :merge isn't a command, which I think is a regression. I'll try and figure that out and at least file a bug.

exchange: add oparg to push so that extensions can wrap pushop

destmerge: extract logic based on branch heads in its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.

destmerge: extract logic based on bookmark into its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.

destupdate: have a generic and extensible way to run each step We want extension to be able to easily override or add new way to select the default update destination. We use the same list + dict approach as in other parts of the code.

destupdate: extract logic based on branch in its own function One of the main goal of having consolidated destination function is to allow extension to play with this logic. We extract sub logic to make is wrapping more practical.