gitk: support link color in the Preferences dialog

As a dark-theme user, I use the Preferences dialog to set colors
for gitk. The only color I cannot change via that dialog is the
link foreground color, which leads to using the default link color
on a dark background that makes it hard to read.

Make the link foreground color also configurable in the Gitk
Preferences dialog's Color tab, so users won't need to dig into
the code/manual to check if it is configurable and can simply set
the color there.

Signed-off-by: Wang Zichong <wangzichong@deepin.org>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

gitk

@@ -11796,7 +11796,7 @@ proc prefspage_general {notebook} {
 
 proc prefspage_colors {notebook} {
     global bgcolor fgcolor ctext diffcolors selectbgcolor markbgcolor
-    global diffbgcolors
+    global diffbgcolors linkfgcolor
     global themeloader
 
     set page [create_prefs_page $notebook.colors]
@@ -11873,6 +11873,11 @@ proc prefspage_colors {notebook} {
         -command [list choosecolor selectbgcolor {} $page [mc "background"]]
     grid x $page.selbgbut $page.selbgsep -sticky w
 
+    label $page.linkfg -padx 40 -relief sunk -background $linkfgcolor
+    ttk::button $page.linkfgbut -text [mc "Link"] \
+        -command [list choosecolor linkfgcolor {} $page [mc "link"]]
+    grid x $page.linkfgbut $page.linkfg -sticky w
+
     grid columnconfigure $page 2 -weight 1
 
     return $page
@@ -11880,7 +11885,7 @@ proc prefspage_colors {notebook} {
 
 proc prefspage_set_colorswatches {page} {
     global bgcolor fgcolor ctext diffcolors selectbgcolor markbgcolor
-    global diffbgcolors
+    global diffbgcolors linkfgcolor
 
     $page.bg configure -background $bgcolor
     $page.fg configure -background $fgcolor
@@ -11891,6 +11896,7 @@ proc prefspage_set_colorswatches {page} {
     $page.hunksep configure -background [lindex $diffcolors 2]
     $page.markbgsep configure -background $markbgcolor
     $page.selbgsep configure -background $selectbgcolor
+    $page.linkfg configure -background $linkfgcolor
 }
 
 proc prefspage_fonts {notebook} {