gitk: Default wrcomcmd to use --pretty=email

This makes the "Write commit to file" context menu option generate a
file that is consumable by 'git am'.
Signed-off-by: NChris Packham <judge.packham@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -11936,7 +11936,7 @@ if { [info exists ::env(GIT_TRACE)] } {
 }
 
 # defaults...
-set wrcomcmd "git diff-tree --stdin -p --pretty"
+set wrcomcmd "git diff-tree --stdin -p --pretty=email"
 
 set gitencoding {}
 catch {