[PATCH v6] gitk: Use git-difftool for external diffs when available

[David Aguilar <davvid@gmail.com>]

git-difftool's '--extcmd=frotz' was added in 1.7.0 and
is the mechanism through which gitk launches the
configured 'extdifftool'.  When 'extdifftool' is
misconfigured an error dialog is used to display
git-difftool's stdout and stderr.

The existing implementation moved into 'proc gitkextdiff'
for use with git < 1.7.0.

One benefit of this change is that gitk's external diff
no longer requires write-access to the current directory.

Signed-off-by: David Aguilar <davvid@gmail.com>
---

Changes since last time:

* Errors are shown using 'proc error_popup'
* The existing code moved into a tidy function

 gitk |   25 +++++++++++++++++++++++++
 1 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/gitk b/gitk
index 1b0e09a..0533baf 100755
--- a/gitk
+++ b/gitk
@@ -3361,6 +3361,7 @@ proc external_diff {} {
     global flist_menu_file
     global diffids
     global extdifftool
+    global git_version
 
     if {[llength $diffids] == 1} {
         # no reference commit given
@@ -3380,6 +3381,30 @@ proc external_diff {} {
         set diffidfrom [lindex $diffids 0]
         set diffidto [lindex $diffids 1]
     }
+    if {[package vcompare $git_version "1.7.0"] < 0} {
+        gitkextdiff $diffidfrom $diffidto
+        return
+    }
+
+    set cmd [list "git" "difftool" "--no-prompt" "--extcmd=$extdifftool"]
+    if {$diffidfrom ne $nullid && $diffidfrom ne $nullid2} {
+        lappend cmd $diffidfrom
+    }
+    if {$diffidto ne $nullid && $diffidto ne $nullid2} {
+        lappend cmd $diffidto
+    }
+    lappend cmd "--" $flist_menu_file
+
+    set pipe [open |$cmd r]
+    set stdout [read $pipe]
+    if {[catch {close $pipe} stderr] != 0} {
+        error_popup "git-difftool: $stdout $stderr"
+    }
+}
+
+proc gitkextdiff {diffidfrom diffidto} {
+    global flist_menu_file
+    global extdifftool
 
     # make sure that several diffs wont collide
     set diffdir [gitknewtmpdir]
-- 
1.7.1.rc2.5.gddd02