Guard against unloaded plugin in runtime files

This can happen if g:loaded_fugitive is set to disable the plugin, or in
a botched install.

ftplugin/fugitiveblame.vim

@@ -1,4 +1,4 @@
-if exists("b:did_ftplugin") || !exists('*fugitive#BlameFileType')
+if exists("b:did_ftplugin") || !exists("*FugitiveGitDir")
   finish
 endif
 let b:did_ftplugin = 1

syntax/fugitiveblame.vim

@@ -1,4 +1,4 @@
-if exists("b:current_syntax")
+if exists("b:current_syntax") || !exists("*FugitiveGitDir")
   finish
 endif