git-gui: Display message box when we cannot find git in $PATH

If we cannot find the git executable in the user's $PATH then
we cannot function correctly.  Because we need that to get the
version so we can load our library correctly we cannot rely on
the library function "error_popup" here, as this is all running
before the library path has been configured, so error_popup is
not available to us.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -498,7 +498,11 @@ proc rmsel_tag {text} {
 set _git  [_which git]
 if {$_git eq {}} {
 	catch {wm withdraw .}
-	error_popup "Cannot find git in PATH."
+	tk_messageBox \
+		-icon error \
+		-type ok \
+		-title [mc "git-gui: fatal error"] \
+		-message [mc "Cannot find git in PATH."]
 	exit 1
 }