gitk: Use --textconv to generate diff text

For the most part gitk's focus is on showing history and changes in
a human readable form.  For this reason, it makes sense to generate
the patch text in the diff view using --textconv so that textconv
drivers are used if they are defined.

gitk can also generate patches, but we do not use --textconv because
such patches could not be applied.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -7268,7 +7268,7 @@ proc getblobdiffs {ids} {
     global limitdiffs vfilelimit curview
     global diffencoding targetline diffnparents
 
-    set cmd [diffcmd $ids "-p -C --cc --no-commit-id -U$diffcontext"]
+    set cmd [diffcmd $ids "-p --textconv -C --cc --no-commit-id -U$diffcontext"]
     if {$ignorespace} {
 	append cmd " -w"
     }