[PATCH RESEND2] git gui: Use git diff --submodule when available

[Jens Lehmann <Jens.Lehmann@web.de>]

Changed the use of submodule summary to diff --submodule because the
implementation in C is much faster than the submodule script. Also a test
has been added to make sure that the underlying git supports the diff
option --submodule (which was introduced in 1.6.6).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
---

Did not hear anything about my last resend on December 28th, so i try
again.

To explain what i mean by "much faster", here are the numbers (best of
three) for a small repo with a changed submodule (git checkout HEAD^)
under Linux:


time git submodule summary
<snip>

real	0m0.219s
user	0m0.050s
sys	0m0.111s


time git diff --submodule
<snip>

real	0m0.012s
user	0m0.003s
sys	0m0.009s


In my experience this patch changes the user experience from
"a noticeable delay" to "instantaneous".


 git-gui/lib/diff.tcl |   21 +++++++++------------
 1 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/git-gui/lib/diff.tcl b/git-gui/lib/diff.tcl
index bd5d189..0623e3e 100644
--- a/git-gui/lib/diff.tcl
+++ b/git-gui/lib/diff.tcl
@@ -281,6 +281,14 @@ proc start_show_diff {cont_info {add_opts {}}} {
 		}
 	}

+	if {[git-version >= "1.6.6"]} {
+		if {[string match {160000 *} [lindex $s 2]]
+		    || [string match {160000 *} [lindex $s 3]]} {
+			set is_submodule_diff 1
+			lappend cmd --submodule
+		}
+	}
+
 	lappend cmd -p
 	lappend cmd --no-color
 	if {$repo_config(gui.diffcontext) >= 1} {
@@ -296,16 +304,6 @@ proc start_show_diff {cont_info {add_opts {}}} {
 		lappend cmd $path
 	}

-	if {[string match {160000 *} [lindex $s 2]]
-        || [string match {160000 *} [lindex $s 3]]} {
-		set is_submodule_diff 1
-		if {$w eq $ui_index} {
-			set cmd [list submodule summary --cached -- $path]
-		} else {
-			set cmd [list submodule summary --files -- $path]
-		}
-	}
-
 	if {[catch {set fd [eval git_read --nice $cmd]} err]} {
 		set diff_active 0
 		unlock_index
@@ -387,8 +385,7 @@ proc read_diff {fd cont_info} {
 			}
 		} elseif {$is_submodule_diff} {
 			if {$line == ""} continue
-			if {[regexp {^\* } $line]} {
-				set line [string replace $line 0 1 {Submodule }]
+			if {[regexp {^Submodule } $line]} {
 				set tags d_@
 			} else {
 				set op [string range $line 0 2]
-- 
1.6.6.rc2.274.g2cee7