This makes it easier to copy diff hunks and paste them into code. We only strip
the prefixes if the copied lines are either all '+' or all '-' (possibly
including context lines), otherwise we keep them. We also keep them when parts
of a hunk header is included in the selection; this is useful for copying a diff
hunk and pasting it into a github comment, for example.
A not-quite-correct edge case is when you select the '--- a/file.txt' line of a
diff header on its own; in this case we copy it as '-- a/file.txt' (same for the
'+++' line). This is probably uncommon enough that it's not worth fixing (it's
not trivial to fix because we don't know that we're in a header).
