gitk: added external diff file rename detection

* If a file was renamed between commits and an external diff is started
  through gitk on the THE ORIGINAL FILE NAME (not the renamed one),
  gitk was unable to open the renamed file in the external diff editor.
  It failed to fetch the renamed file from git, because it fetched it
  with the original path in contrast to using the renamed path
* gitk now detects the rename and opens the external diff with the
  original and the RENAMED file instead of no file (it is able to
  fetch the renamed file now from git with the renamed path/filename)
* Since git doesn't destinguish between move or rename this also works
  for moved files
* Showing the renamed file doesn't work when THE RENAMED FILE is selected
  in gitk and an external diff ist started on that file, because the
  selected file is not renamed in that commit. It already IS the renamed
  file.

Signed-off-by: Tobias Boeesch <tobias.boesch@miele.com>

Author: ToBoMi

gitk-git/gitk

@@ -3666,6 +3666,27 @@ proc external_diff_get_one_file {diffid filename diffdir} {
                "revision $diffid"]
 }
 
+proc check_for_renames_in_diff {filepath} {
+    global ctext
+
+    set renamed_filenames [list {}]
+    set filename [file tail $filepath]
+    set rename_from_text_identifier_length 12
+    set rename_to_text_identifier_length 10
+    set reg_expr_rename_from {^rename from (.*$filename)}
+    set reg_expr_rename_from [subst -nobackslashes -nocommands $reg_expr_rename_from]
+    set reg_expr_rename_to {^rename to (.*)}
+    set rename_from_text_index [$ctext search -elide -regexp -- $reg_expr_rename_from 0.0]
+    if { ($rename_from_text_index != {})} {
+        set rename_to_text_index [$ctext search -elide -regexp -- $reg_expr_rename_to $rename_from_text_index]
+        if { ($rename_from_text_index != {}) && ($rename_to_text_index != {}) } {
+            lappend renamed_filenames [$ctext get "$rename_from_text_index + $rename_from_text_identifier_length chars" "$rename_from_text_index lineend"]
+            lappend renamed_filenames [$ctext get "$rename_to_text_index + $rename_to_text_identifier_length chars" "$rename_to_text_index lineend"]
+        }
+    }
+    return $renamed_filenames
+}
+
 proc external_diff {} {
     global nullid nullid2
     global flist_menu_file
@@ -3696,8 +3717,16 @@ proc external_diff {} {
     if {$diffdir eq {}} return
 
     # gather files to diff
-    set difffromfile [external_diff_get_one_file $diffidfrom $flist_menu_file $diffdir]
-    set difftofile [external_diff_get_one_file $diffidto $flist_menu_file $diffdir]
+    set renamed_filenames [check_for_renames_in_diff $flist_menu_file]
+    set rename_from_filename [lindex $renamed_filenames 1]
+    set rename_to_filename [lindex $renamed_filenames 2]
+    if { ($rename_from_filename != {}) && ($rename_to_filename != {}) } {
+        set difffromfile [external_diff_get_one_file $diffidfrom $rename_from_filename $diffdir]
+        set difftofile [external_diff_get_one_file $diffidto $rename_to_filename $diffdir]
+    } else {
+        set difffromfile [external_diff_get_one_file $diffidfrom $flist_menu_file $diffdir]
+        set difftofile [external_diff_get_one_file $diffidto $flist_menu_file $diffdir]
+    }
 
     if {$difffromfile ne {} && $difftofile ne {}} {
         set cmd [list [shellsplit $extdifftool] $difffromfile $difftofile]