internal/diff: flush to disk before diffing
internal/diff: flush to disk before diffing

Flush strings to disk before diffing files.

Change-Id: I91c47bfe352ee833a164fa7de00341e96498bc96
Reviewed-on: https://go-review.googlesource.com/39235
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/perf,golang/perf