gitk: Indent word-wrapped lines in commit display header

In the cases where the lines starting with Precedes:, Follows: and
Branches: in the commit display are long enough to be word-wrapped,
this adds a 1cm margin on the left of the wrapped lines, to make
the display more readable.  Suggested by Stephen Rothwell.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -2429,7 +2429,7 @@ proc makewindow {} {
     $ctext tag conf msep -font textfontbold
     $ctext tag conf found -back $foundbgcolor
     $ctext tag conf currentsearchhit -back $currentsearchhitbgcolor
-    $ctext tag conf wwrap -wrap word
+    $ctext tag conf wwrap -wrap word -lmargin2 1c
     $ctext tag conf bold -font textfontbold
 
     .pwbottom add .bleft