git-gui: Cache the GIT_COMMITTER_IDENT value on first sign-off.

Caching the Signed-Off-By line isn't very important (as its not
performance critical).  The major improvement here is that we
now report an error to the user if we can't obtain their name
from git-var.

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

git-gui

@@ -1398,19 +1398,28 @@ proc do_include_all {} {
 	}
 }
 
+set GIT_COMMITTER_IDENT {}
+
 proc do_signoff {} {
-	global ui_comm
+	global ui_comm GIT_COMMITTER_IDENT
 
-	catch {
-		set me [exec git var GIT_COMMITTER_IDENT]
-		if {[regexp {(.*) [0-9]+ [-+0-9]+$} $me me name]} {
-			set str "Signed-off-by: $name"
-			if {[$ui_comm get {end -1c linestart} {end -1c}] != $str} {
-				$ui_comm insert end "\n"
-				$ui_comm insert end $str
-				$ui_comm see end
-			}
+	if {$GIT_COMMITTER_IDENT == {}} {
+		if {[catch {set me [exec git var GIT_COMMITTER_IDENT]} err]} {
+			error_popup "Unable to obtain your identity:\n$err"
+			return
 		}
+		if {![regexp {^(.*) [0-9]+ [-+0-9]+$} \
+			$me me GIT_COMMITTER_IDENT]} {
+			error_popup "Invalid GIT_COMMITTER_IDENT:\n$me"
+			return
+		}
+	}
+
+	set str "Signed-off-by: $GIT_COMMITTER_IDENT"
+	if {[$ui_comm get {end -1c linestart} {end -1c}] != $str} {
+		$ui_comm insert end "\n"
+		$ui_comm insert end $str
+		$ui_comm see end
 	}
 }