Only override 'guitablabel' if user hasn't already set it

If guitablabel is already set in .vimrc, don't have the MacVim default
gvimrc override it. Even though Vim's docs recommend putting GUI
settings in gvimrc, that's mostly for technical reasons, and MacVim
should still aim to respect settings that a user has already set. This
also makes it behave more similarly with Vim (you can set 'guitablabel'
in vimrc or gvimrc and it would still work).

This was previously done in #635 but reverted, but #899 raised this
issue again.

Close #899

Author: ychin

runtime/doc/gui_mac.txt

@@ -105,9 +105,11 @@ dropping two more, and switching back to the first tab would cause weird
 strings like "((3) of 2)" to appear in the window title.
 
 							*macvim-tablabel*
-Tab labels only show the tail of the file name to make the tabs more readable
-when editing files in deeply nested folders.  Add the line "set guitablabel="
-to your .gvimrc file to revert back to the default Vim tab label.
+By default, tab labels only show the tail of the file name to make the tabs
+more readable when editing files in deeply nested folders.  Add the line "set
+guitablabel=" to your .gvimrc file to revert back to the default Vim tab
+label.  Setting 'guitablabel' to anything in your .vimrc will also prevent
+this default from taking effect.
 
 							*macvim-options*
 These are the non-standard options that MacVim supports:

src/MacVim/gvimrc

@@ -28,7 +28,9 @@ endif
 
 " To make tabs more readable, the label only contains the tail of the file
 " name and the buffer modified flag.
-set guitablabel=%M%t
+if empty(&guitablabel)
+  set guitablabel=%M%t
+endif
 
 " Send print jobs to Preview.app.  This does not delete the temporary ps file
 " that is generated by :hardcopy.