buffer/settings: Don't use Modified() before we updated origHash

When we have already enabled `fastdirty` but have not updated origHash
yet, we shouldn't use Modified() since it depends on origHash which is
still outdated, and thus returns wrong values.

This fixes the following issue: enable `fastdirty`, modify the buffer,
save the buffer and disable `fastdirty` -> micro wrongly reports the
buffer as modified (whereas it has just been saved).

Note that this fix, though, also causes a regression: e.g. if we run
`set fastdirty false` while fastdirty is already disabled, micro may
unexpectedly report a non-modified buffer as modified (in the case if
isModified is true but the buffer it actually not modified, since its
md5 sum matches and fastdirty is disabled), since this fix assumes that
since we are disabling fastdirty, it has been enabled. This shall be
fixed by PR #3343 which makes `set` do nothing if the option value
doesn't change.

internal/buffer/settings.go

@@ -12,7 +12,7 @@ func (b *Buffer) SetOptionNative(option string, nativeValue interface{}) error {
 
 	if option == "fastdirty" {
 		if !nativeValue.(bool) {
-			if !b.Modified() {
+			if !b.isModified {
 				e := calcHash(b, &b.origHash)
 				if e == ErrFileTooLarge {
 					b.Settings["fastdirty"] = true