git-gui: Gracefully handle bad TCL_PATH at compile time

Petr Baudis pointed out the main git.git repository's Makefile dies
now if git-gui 0.7.0-rc1 or later is being used and TCL_PATH was not
set to a working tclsh program path.  This breaks people who may have
a working build configuration today and suddenly upgrade to the latest
git release.

The tclIndex is required for git-gui to load its associated lib files,
but using the Tcl auto_load procedure to source only the files we need
is a performance optimization.  We can emulate the auto_load by just
source'ing every file in that directory, assuming we source class.tcl
first to initialize our crude class system.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -28,7 +28,34 @@ set oguilib {@@GITGUI_LIBDIR@@}
 if {[string match @@* $oguilib]} {
 	set oguilib [file join [file dirname [file normalize $argv0]] lib]
 }
-set auto_path [concat [list $oguilib] $auto_path]
+set idx [file join $oguilib tclIndex]
+catch {
+	set fd [open $idx r]
+	if {[gets $fd] eq {# Autogenerated by git-gui Makefile}} {
+		set idx [list]
+		while {[gets $fd n] >= 0} {
+			if {$n ne {} && ![string match #* $n]} {
+				lappend idx $n
+			}
+		}
+	} else {
+		set idx {}
+	}
+	close $fd
+}
+if {$idx ne {}} {
+	set loaded [list]
+	foreach p $idx {
+		if {[lsearch -exact $loaded $p] >= 0} continue
+		puts $p
+		source [file join $oguilib $p]
+		lappend loaded $p
+	}
+	unset loaded p
+} else {
+	set auto_path [concat [list $oguilib] $auto_path]
+}
+unset -nocomplain fd idx
 
 if {![catch {set _verbose $env(GITGUI_VERBOSE)}]} {
 	unset _verbose