docs: correct ORIG_HEAD example in "git merge" documentation

Documentation for git-merge incorrectly notes that
tip of the current branch on ascii diagram is C,
while it is actually G (current branch is master,
HEAD on diagram is G).

Additionally diagrams on the page are adjusted
to use spaces instead of tabs, so that they align
regardless of tab size. This is in line with
diagrams on other git documentation pages.

Signed-off-by: Timur Sultanaev <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: strowk

Documentation/git-merge.adoc

@@ -28,8 +28,8 @@ Assume the following history exists and the current branch is
 `master`:
 
 ------------
-	  A---B---C topic
-	 /
+          A---B---C topic
+         /
     D---E---F---G master
 ------------
 
@@ -38,11 +38,11 @@ Then `git merge topic` will replay the changes made on the
 its current commit (`C`) on top of `master`, and record the result
 in a new commit along with the names of the two parent commits and
 a log message from the user describing the changes. Before the operation,
-`ORIG_HEAD` is set to the tip of the current branch (`C`).
+`ORIG_HEAD` is set to the tip of the current branch (`G`).
 
 ------------
-	  A---B---C topic
-	 /         \
+          A---B---C topic
+         /         \
     D---E---F---G---H master
 ------------