git-gui: Cleanup usage of gitdir global variable.

git-gui: Cleanup usage of gitdir global variable.

The gitdir global variable is essentially read-only, and is used rather
frequently.  So are appname and reponame.  Needing to constantly declare
'global appname' just so we can access the value as $appname is downright
annoying and redundant.  So instead I'm declaring these as procedures and
changing all uses to invoke the procedure rather than access the global
directly.

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