gitk: Simplify calculation of gitdir

Since 5024baa ([PATCH] Make gitk work when launched in a subdirectory,
2007-01-09), gitk has used 'git rev-parse --git-dir' to find the .git
directory. However, gitk still first checks for the $GIT_DIR
environment variable and that the value returned from git-rev-parse
does not point to a file. Since git-rev-parse does both of these
checks already, the checks can safely be removed from gitk. This makes
the gitdir procedure small enough to inline.

This cleanup introduces a UI regression in that the error message will
now be "Cannot find a git repository here." even in the case where
GIT_DIR points to a file, for which the error message was previously
"Cannot find the git directory \"%s\".". It should be noted, though,
that even before this patch, 'gitk --git-dir=path/to/some/file' would
give the former error message.

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

gitk

@@ -9,15 +9,6 @@ exec wish "$0" -- "$@"
 
 package require Tk
 
-proc gitdir {} {
-    global env
-    if {[info exists env(GIT_DIR)]} {
-	return $env(GIT_DIR)
-    } else {
-	return [exec git rev-parse --git-dir]
-    }
-}
-
 # A simple scheduler for compute-intensive stuff.
 # The aim is to make sure that event handlers for GUI actions can
 # run at least every 50-100 ms.  Unfortunately fileevent handlers are
@@ -11523,14 +11514,10 @@ setui $uicolor
 setoptions
 
 # check that we can find a .git directory somewhere...
-if {[catch {set gitdir [gitdir]}]} {
+if {[catch {set gitdir [exec git rev-parse --git-dir]}]} {
     show_error {} . [mc "Cannot find a git repository here."]
     exit 1
 }
-if {![file isdirectory $gitdir]} {
-    show_error {} . [mc "Cannot find the git directory \"%s\"." $gitdir]
-    exit 1
-}
 
 set selecthead {}
 set selectheadid {}