gitk: Run 'git rev-parse --git-dir' only once

It seems like gitk has been setting the global variable 'gitdir' at
startup since aa81d97 (gitk: Fix Update menu item, 2006-02-28).  It
should therefore no longer be necessary to call the procedure with the
same name (more than once to set the global variable).  Remove the
other call sites and use the global variable instead.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -9059,6 +9059,7 @@ proc exec_citool {tool_args {baseid {}}} {
 proc cherrypick {} {
     global rowmenuid curview
     global mainhead mainheadid
+    global gitdir
 
     set oldhead [exec git rev-parse HEAD]
     set dheads [descheads $rowmenuid]
@@ -9087,7 +9088,7 @@ proc cherrypick {} {
 			conflict.\nDo you wish to run git citool to\
 			resolve it?"]]} {
 		# Force citool to read MERGE_MSG
-		file delete [file join [gitdir] "GITGUI_MSG"]
+		file delete [file join $gitdir "GITGUI_MSG"]
 		exec_citool {} $rowmenuid
 	    }
 	} else {
@@ -9453,14 +9454,15 @@ proc refill_reflist {} {
 proc getallcommits {} {
     global allcommits nextarc seeds allccache allcwait cachedarcs allcupdate
     global idheads idtags idotherrefs allparents tagobjid
+    global gitdir
 
     if {![info exists allcommits]} {
 	set nextarc 0
 	set allcommits 0
 	set seeds {}
 	set allcwait 0
 	set cachedarcs 0
-	set allccache [file join [gitdir] "gitk.cache"]
+	set allccache [file join $gitdir "gitk.cache"]
 	if {![catch {
 	    set f [open $allccache r]
 	    set allcwait 1