Check for the existence of the git directory on startup.

Check that $GIT_DIR (or .git, if GIT_DIR is not set) is a directory.
This means we can give a more informative error message if the user
runs gitk somewhere that isn't a git repository.

gitk

@@ -8,10 +8,20 @@ exec wish "$0" -- "${1+$@}"
 # either version 2, or (at your option) any later version.
 
 proc getcommits {rargs} {
-    global commits commfd phase canv mainfont
+    global commits commfd phase canv mainfont env
     global startmsecs nextupdate
     global ctext maincursor textcursor leftover
 
+    # check that we can find a .git directory somewhere...
+    if {[info exists env(GIT_DIR)]} {
+	set gitdir $env(GIT_DIR)
+    } else {
+	set gitdir ".git"
+    }
+    if {![file isdirectory $gitdir]} {
+	error_popup "Cannot find the git directory \"$gitdir\"."
+	exit 1
+    }
     set commits {}
     set phase getcommits
     set startmsecs [clock clicks -milliseconds]