git-gui: Force an update-index --refresh on unchanged files.

git-gui: Force an update-index --refresh on unchanged files.

Its possible for external programs to update file modification dates of
many files within a repository.  I've seen this on Windows with a popular
virus scanner, sadly enough.  If the user has Trust File Modification
Timestamp enabled and the virus scanner touches a large number of files
it can be annoying trying to clear them out of the 'Changed But Not
Updated' file list by clicking on them one at a time to load the diff.

So now we force a rescan as soon as one such file is found, and for
just that rescan we disable the Trust File Modification Timestamp option
thereby allowing Git to update the modification dates in the index.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>