git-gui: Remove forced rescan of stat-dirty files.

It is possible that stat information of tracked files is modified without actually modifying the content. Plumbing commands would detect such files as modified, so that Git GUI runs `git update-info --refresh` in order to synchronize the cached stat info with the reality. However, this can be an expensive operation in large repositories. As remediation, e534f3a88676 (git-gui: Allow the user to disable update-index --refresh during rescan, 2006-11-07) introduced an option to skip the expensive part. The option was named "trust file modification timestamp". But the catch is that sometimes file timestamps can't be trusted. In this case, a file would remain listed in Unstaged Changes although there are no changes. So 16403d0b1f9d (git-gui: Refresh a file if it has an empty diff, 2006-11-11) introduced a popup message informing the user about the situation and then removed the file from the Unstaged Changes list. Now users had to click away the message box for every file that was stat-dirty. Under the assumption that a file in such a state is not the only one, 124355d32c06 (git-gui: Always start a rescan on an empty diff, 2007-01-22) introduced a forced (potentially expensive) refresh that would de-list all stat-dirty files after the first notification was dismissed. Along came 6c510bee2013 (Lazy man's auto-CRLF, 2007-02-13) in Git. It introduced a new case where a file in the worktree can have no essential differences to the staged version, but still be detected as modified by plumbing commands. This time, however, the index cannot be synchronized fully by `git update-index --refresh`, so that the file remains listed in Unstaged Changes until it is staged manually. Needless to say that the message box now becomes an annoyance, because it must be dismissed every time an affected file is selected, and the file remains listed nevertheless. Remove the message box. Write the notice that no differences were found in the diff panel instead. Also include a link that, when clicked, initiates the rescan. With this scheme, the rescan does not happen automatically anymore, but requires an additional click. (This is now two clicks in total for users who encounter stat-dirty files after enabling the "trust file modification timestamps" option.) However, users whom the rescan does not help (autocrlf-related dirty files) save half the clicks because there is no message box to dismiss. Signed-off-by: Johannes Sixt <j6t@kdbg.org>