At the interactive conflict prompt, make the 'show-diff' option show
more sensible diffs.

We used to always show a diff between 'base' and 'merged', which shows how
the pristine version of the file differs from the merged version.

However, with conflicts recorded during 'svn merge', the 'base' version
of the file is the 3-way merge base, which can be a file on a different
branch. Thus the diff would be unnecessarily large, and show changes that
don't actually appear when the user runs 'svn diff' in the working copy.

In some cases the 'show-diff' option was entirely useless to help identify
the incoming changes (e.g. try a merge of r1438602 into a 1.7.x@1438875
and select the 'show-diff' option during the conflict prompt).

To fix this, show the diff between 'mine' and 'merged' during merges,
and show the diff between 'theirs' and 'merged' during updates (this
doesn't actually change behaviour, since 'base' matches 'theirs' during
updates, but eliminates an unnecessary non-NULL check for the 'base' abspath).

A simple example of the resuling visible change in behaviour:

Consider the file ^/trunk/alpha which changed its content from 'alpha'
to 'foo' in r3, and a ^/branch off trunk@2, where 'alpha' was changed from
'alpha' to 'boo' in r4 on the branch.
When merging trunk into a branch there is a conflict, and we used to show:

  --- /tmp/svn-sOVNQg     Sat Jan 26 12:00:28 2013
  +++ /tmp/svn-sandbox/branch/.svn/tmp/alpha.tmp  Sat Jan 26 12:00:28 2013
  @@ -1 +1,5 @@
  -alpha
  +<<<<<<< .working
  +boo
  +=======
  +foo
  +>>>>>>> .merge-right.r4

This is actually diffing 'alpha' as it appeared on trunk in r2 with
the merged file, showing the content which is already present on the
branch as newly added. This is clearly confusing (and even more confusing
in real life with large files).

Now, diff-full shows:

  --- /tmp/svn-sandbox/branch/alpha.working       Sat Jan 26 12:03:25 2013
  +++ /tmp/svn-sandbox/branch/alpha       Sat Jan 26 12:03:25 2013
  @@ -1 +1,5 @@
  +<<<<<<< .working
   boo
  +=======
  +foo
  +>>>>>>> .merge-right.r4

which clearly shows the change brought in during the merge, and matches
what 'svn diff' shows when we postpone the conflict.

* subversion/svn/conflict-callbacks.c
  (show_diff): Tweak docstring and implementation as per above.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1438879 13f79535-47bb-0310-9956-ffa450edef68