merge-ort: make informational messages from recursive merges clearer

This is another simple change with a long explanation...

merge-recursive and merge-ort are both based on the same recursive idea:
if there is more than one merge base, merge the merge bases (which may
require first merging the merge bases of the merges bases, etc.).  The
depth of the inner merge is recorded via a variable called "call_depth",
which we'll bring up again later.  Naturally, the inner merges
themselves can have conflicts and various messages generated about those
files.

merge-recursive immediately prints to stdout as it goes, at the risk of
printing multiple conflict notices for the same path separated far apart
from each other with many intervenining conflict notices for other paths
between them.  And this is true even if there are no inner merges
involved.  An example of this was given in [1] and apparently caused
some confusion:

    CONFLICT (rename/add): Rename A->B in HEAD. B added in otherbranch
    ...dozens of conflicts for OTHER paths...
    CONFLICT (content): Merge conflicts in B

In contrast, merge-ort collects messages and stores them by path so that
it can print them grouped by path.  Thus, the same case handled by
merge-ort would have output of the form:

    CONFLICT (rename/add): Rename A->B in HEAD. B added in otherbranch
    CONFLICT (content): Merge conflicts in B
    ...dozens of conflicts for OTHER paths...

This is generally helpful, but does make a separate bug more
problematic.  In particular, while merge-recursive might report the
following for a recursive merge:

      Auto-merging dir.c
      Auto-merging midx.c
      CONFLICT (content): Merge conflict in midx.c
    Auto-merging diff.c
    Auto-merging dir.c
    CONFLICT (content): Merge conflict in dir.c

merge-ort would instead report:

    Auto-merging diff.c
    Auto-merging dir.c
    Auto-merging dir.c
    CONFLICT (content): Merge conflict in dir.c
    Auto-merging midx.c
    CONFLICT (content): Merge conflict in midx.c

The fact that messages for the same file are together is probably
helpful in general, but with the indentation missing for the inner
merge it unfortunately serves to confuse.  This probably would lead
users to wonder:

  * Why is Git reporting that "dir.c" is being merged twice?
  * If midx.c has conflicts, why do I not see any when I open up the
    file and why are no conflicts shown in the index?

Fix this output confusion by changing the output to clearly
differentiate the messages for outer merges from the ones for inner
merges, changing the above output from merge-ort to:

    Auto-merging diff.c
      From inner merge:  Auto-merging dir.c
    Auto-merging dir.c
    CONFLICT (content): Merge conflict in dir.c
      From inner merge:  Auto-merging midx.c
      From inner merge:  CONFLICT (content): Merge conflict in midx.c

(Note: the number of spaces after the 'From inner merge:' is
2*call_depth).

One other thing to note here, that I didn't notice until typing up this
commit message, is that merge-recursive does not print any messages from
the inner merges by default; the extra verbosity has to be requested.
merge-ort currently has no verbosity controls and always prints these.
We may also want to change that, but for now, just make the output
clearer with these extra markings and indentation.

[1] https://lore.kernel.org/git/CAGyf7-He4in8JWUh9dpAwvoPkQz9hr8nCBpxOxhZEd8+jtqTpg@mail.gmail.com/

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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