gitk: Put temporary directory inside .git

When running "External diff" from gitk, the "from" and "to" files will
first be copied into a directory that is currently
".git/../.gitk-tmp.$pid".  When gitk is closed, the directory is
deleted. When the work tree is not at ".git/.." (which is supported
since the previous commit), that directory may not even be git-related
and it does not seem unlikely that permissions may not allow the
temporary directory to be created there.  Move the directory inside
.git instead.

This introduces a regression in the case that the .git directory
is readonly, but .git/.. is writeable.
Signed-off-by: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -3331,8 +3331,7 @@ proc gitknewtmpdir {} {
     global diffnum gitktmpdir gitdir
 
     if {![info exists gitktmpdir]} {
-	set gitktmpdir [file join [file dirname $gitdir] \
-			    [format ".gitk-tmp.%s" [pid]]]
+	set gitktmpdir [file join $gitdir [format ".gitk-tmp.%s" [pid]]]
 	if {[catch {file mkdir $gitktmpdir} err]} {
 	    error_popup "[mc "Error creating temporary directory %s:" $gitktmpdir] $err"
 	    unset gitktmpdir