Disable patch commands for commits on a different branch

[stefanhaller]

In the commits panel it is possible to show commits of all branches (by doing <c-l>, "Toggle show whole git graph"). You can then show the files of a commit that is not part of the current branch, build a custom patch from it, and then perform one of the commands

• Remove patch from original commit
• Move patch out into index
• Move patch into new commit
• Move patch to selected commit (if a different commit is selected)

These will all fail with hard-to-understand errors. We should disable them when the commit is not part of the current branch (or the target commit, in case of "Move patch to selected commit").

Conversely, we may want to enable these commits if the patch was created from the branches panel, and the commit is part of the current branch. I'm not really sure this is useful, but it came up here and caused some confusion.

No matter what we decide about that last one, we should always include all commands in the menu, and strike out the ones that are not applicable, with tooltips explaining why.

[stefanhaller]

There are many other places in the code that rely on the assumption that all commits shown in the commits panel are part of the current branch. For example, <c-j> and <c-k> to move a commit up/down will fail with the same cryptic error when performed on a commit that's on a different branch.