Revert "git-gui: Don't create empty (same tree as parent) commits."

This reverts commit 51bd9d7b8c.
It introduced a git invocation that involves ^{tree}. It seems that MSYS's
wish does some quoting for Bourne shells, in particular, escape the first
'{', but then it uses cmd.exe to run the command, which does not remove
the backslash, so that the resulting ref expression ends up in git's guts
as unrecognizable garbage.

git-gui/git-gui.sh

@@ -1269,24 +1269,6 @@ proc commit_committree {fd_wt curHEAD msg} {
 		return
 	}
 
-	# -- Verify this wasn't an empty change.
-	#
-	if {$commit_type eq {normal}} {
-		set old_tree [git rev-parse "$PARENT^{tree}"]
-		if {$tree_id eq $old_tree} {
-			info_popup {No changes to commit.
-
-No files were modified by this commit and it
-was not a merge commit.
-
-A rescan will be automatically started now.
-}
-			unlock_index
-			rescan {set ui_status_value {No changes to commit.}}
-			return
-		}
-	}
-
 	# -- Build the message.
 	#
 	set msg_p [gitdir COMMIT_EDITMSG]