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

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>