git-gui: Avoid unnecessary global statements when possible

Running global takes slightly longer than just accessing the variable
via its package name, especially if the variable is just only once in
the procedure, or isn't even used at all in the procedure.  So this is
a minor cleanup for some of our commonly invoked procedures.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -154,12 +154,10 @@ proc gitexec {args} {
 }
 
 proc reponame {} {
-	global _reponame
-	return $_reponame
+	return $::_reponame
 }
 
 proc is_MacOSX {} {
-	global tcl_platform tk_library
 	if {[tk windowingsystem] eq {aqua}} {
 		return 1
 	}
@@ -167,17 +165,16 @@ proc is_MacOSX {} {
 }
 
 proc is_Windows {} {
-	global tcl_platform
-	if {$tcl_platform(platform) eq {windows}} {
+	if {$::tcl_platform(platform) eq {windows}} {
 		return 1
 	}
 	return 0
 }
 
 proc is_Cygwin {} {
-	global tcl_platform _iscygwin
+	global _iscygwin
 	if {$_iscygwin eq {}} {
-		if {$tcl_platform(platform) eq {windows}} {
+		if {$::tcl_platform(platform) eq {windows}} {
 			if {[catch {set p [exec cygpath --windir]} err]} {
 				set _iscygwin 0
 			} else {