gitk: Switch to patch mode when searching for line origin

If the "Show origin of this line" is started from tree mode,
it still shows the result in tree mode, which I suppose not
what user expects to see.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -3871,7 +3871,7 @@ proc read_line_source {fd inst} {
 	    set id $nullid2
 	}
 	if {[commitinview $id $curview]} {
-	    selectline [rowofcommit $id] 1 [list $fname $lnum]
+	    selectline [rowofcommit $id] 1 [list $fname $lnum] 1
 	} else {
 	    error_popup [mc "That line comes from commit %s, \
 			     which is not in this view" [shortids $id]]
@@ -7162,7 +7162,7 @@ proc make_idmark {id} {
     $canv raise $t
 }
 
-proc selectline {l isnew {desired_loc {}}} {
+proc selectline {l isnew {desired_loc {}} {switch_to_patch 0}} {
     global canv ctext commitinfo selectedline
     global canvy0 linespc parents children curview
     global currentid sha1entry
@@ -7188,6 +7188,10 @@ proc selectline {l isnew {desired_loc {}}} {
 	setcanvscroll
     }
 
+    if {$cmitmode ne "patch" && $switch_to_patch} {
+        set cmitmode "patch"
+    }
+
     set y [expr {$canvy0 + $l * $linespc}]
     set ymax [lindex [$canv cget -scrollregion] 3]
     set ytop [expr {$y - $linespc - 1}]