Overview

This PR adds a diff.branch command that displays two two-way diff previews between the common ancestor of two branches, and each branch head:

Additionally, if the UCM_DIFFTOOL environment variable is set, it will be executed with variables $LOCAL, $REMOTE, $BASE, and $MERGED substituted, as described in https://git-scm.com/docs/git-difftool.

Note: I haven't found a good diff tool that actually looks at $BASE to produce a three-way diff. One popular one, delta, seems to only render three-way diffs if it's given a file with zdiff3-style markers in it. So, we may want to do something like that instead of the current implementation.

Nonetheless, with this PR, you can at least get an off-the-shelf tool to show a two-way diff between two branch heads, which might look like:

Test coverage

I've added a transcript to capture the diff.branch output.

Loose ends

• The diff.branch command can't currently produce a diff between two branches that don't have a shared history. In this case, the plan is to treat the left argument as the LCA, so that (e.g.) diff.branch old_release new_release will show the changes in new_release relative to old_release. However, that work is left for a future PR.
• The diff.branch command doesn't currently render changes to library dependencies. That's not hard to do, I just left it off this PR and intend to follow up.
• The output message of diff.branch doesn't mention UCM_DIFFTOOL (i.e. "Tip: consider setting the UCM_DIFFTOOL environment variable as documented ") if it isn't set. Should it?
• If UCM_DIFFTOOL process is configured but fails, that also is just silenced, not included in an output message.