don't update sync time if no files have changed (and thus avoid re-indexing after every sync attempt)
don't update sync time if no files have changed
(and thus avoid re-indexing after every sync attempt)

R=rsc
http://go/go-review/1016010

tianon/debian-golang,pombredanne/go-deleteme,tianon/debian-golang,apparentlymart/golang,h8liu/golang,bdwalton/debian-golang,dsriram/golang,bdwalton/debian-golang,tv42/old-go,bdwalton/debian-golang,tv42/old-go,tianon/debian-golang,pombredanne/go-deleteme,ssrl/go,bdwalton/debian-golang,tv42/old-go,tv42/old-go,apparentlymart/golang,h8liu/golang,ssrl/go,pombredanne/go-deleteme,nightlyone/golang,tianon/debian-golang,ssrl/go,tianon/debian-golang,nightlyone/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tianon/debian-golang,dsriram/golang,tv42/old-go,bdwalton/debian-golang,h8liu/golang,nightlyone/golang,gnanderson/go,h8liu/golang,gnanderson/go,apparentlymart/golang,ssrl/go,ssrl/go,ssrl/go,pombredanne/go-deleteme,nightlyone/golang,nightlyone/golang,bdwalton/debian-golang,gnanderson/go,apparentlymart/golang,dsriram/golang,nightlyone/golang,bdwalton/debian-golang,apparentlymart/golang,pombredanne/go-deleteme,tianon/debian-golang,h8liu/golang,ssrl/go,h8liu/golang,dsriram/golang,bdwalton/debian-golang,pombredanne/go-deleteme,tv42/old-go,h8liu/golang,gnanderson/go,gnanderson/go,tianon/debian-golang,apparentlymart/golang,nightlyone/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang