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>

git-gui.sh

@@ -335,7 +335,7 @@ proc PARENT {} {
 	return $empty_tree
 }
 
-proc rescan {after} {
+proc rescan {after {honor_trustmtime 1}} {
 	global HEAD PARENT MERGE_HEAD commit_type
 	global ui_index ui_workdir ui_status_value ui_comm
 	global rescan_active file_states
@@ -366,7 +366,7 @@ proc rescan {after} {
 		$ui_comm edit modified false
 	}
 
-	if {$repo_config(gui.trustmtime) eq {true}} {
+	if {$honor_trustmtime && $repo_config(gui.trustmtime) eq {true}} {
 		rescan_stage2 {} $after
 	} else {
 		set rescan_active 1
@@ -586,17 +586,11 @@ by another application and you currently have
 the Trust File Modification Timestamps option
 enabled, so Git did not automatically detect
 that there are no content differences in this
-file.
-
-This file will now be removed from the modified
-files list, to prevent possible confusion.
-"
-	if {[catch {exec git update-index -- $path} err]} {
-		error_popup "Failed to refresh index:\n\n$err"
-	}
+file."
 
 	clear_diff
 	display_file $path __
+	rescan {set ui_status_value {Ready.}} 0
 }
 
 proc show_diff {path w {lno {}}} {