We want some functions to check if commits are cherry-picks or reverts,
based on information recorded in the commit message.  Namely, substrings
of the form
  This reverts commit ... [reversing\nchanges made to ...]
  cherry picked from commit ...
When these strings are found, get_message_pick() can return that the
specified commit is a cherry-pick or revert and what other commit the
specified commit is a cherry-pick or revert of.

We will use this later to implement --remerge-diff-only, where normal
non-merge commits that are not cherry-picks or reverts are omitted from
log/show output, and cherry-picks or reverts are shown as a diff against
an automatic re-pick/re-revert.  Thus, the new flag will be a way to see
what changes the creator of the commit made in order to complete the
cherry-pick or revert, such as removing conflict markers.

Signed-off-by: Elijah Newren <newren@gmail.com>

--remerge-diff-only is similar to --remerge-diff and implies its
behavior, but additionally will also change how all single-parent
commits are displayed.

If a single parent commit cannot be determined to be either a
cherry-pick or a revert (based on its commit message), then: (a) for
`git log`, the commit will simply be skipped, (b) for `git show`, a
warning that we could not find cherry-pick or revert information for the
commit will be shown.

If a single parent commit can be determined to be a cherry pick or a
revert (and the commit it is a cherry pick or revert of can be found in
the commit message), then --remerge-diff-only will cause that commit to
be automatically reverted or cherry-picked again (but only creating a
new tree, not a new commit), and then the commit will be diffed against
the automatic revert or cherry-pick tree.

Signed-off-by: Elijah Newren <newren@gmail.com>