git-merge-index.txt: wrap shell listing in "----"

The example output of `git merge-index` has been enriched by a second
"column" of helpful comments. When Asciidoctor renders this, the cells
in that second column aren't aligned.

Fix this by marking the example shell session as a code listing by
wrapping it in "----". Also drop some of the horizontal space between
the two columns so that we fit into 80 columns. This changes the
rendering with both AsciiDoc and Asciidoctor. They now render this
identically, nicely aligned, and within 80 columns.

Signed-off-by: Martin Ågren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Martin Ågren

Documentation/git-merge-index.txt

@@ -54,20 +54,24 @@ original is first. But the argument order to the 3-way merge program
 
 Examples:
 
-  torvalds@ppc970:~/merge-test> git merge-index cat MM
-  This is MM from the original tree.			# original
-  This is modified MM in the branch A.			# merge1
-  This is modified MM in the branch B.			# merge2
-  This is modified MM in the branch B.			# current contents
+----
+torvalds@ppc970:~/merge-test> git merge-index cat MM
+This is MM from the original tree.		# original
+This is modified MM in the branch A.		# merge1
+This is modified MM in the branch B.		# merge2
+This is modified MM in the branch B.		# current contents
+----
 
 or
 
-  torvalds@ppc970:~/merge-test> git merge-index cat AA MM
-  cat: : No such file or directory
-  This is added AA in the branch A.
-  This is added AA in the branch B.
-  This is added AA in the branch B.
-  fatal: merge program failed
+----
+torvalds@ppc970:~/merge-test> git merge-index cat AA MM
+cat: : No such file or directory
+This is added AA in the branch A.
+This is added AA in the branch B.
+This is added AA in the branch B.
+fatal: merge program failed
+----
 
 where the latter example shows how 'git merge-index' will stop trying to
 merge once anything has returned an error (i.e., `cat` returned an error