handle missing dep files in concat-deps

* this and Ib4762f5a260035f00b5e68cf45b687fdf02e9c02 reduces a default
build on my machine from 2min25sec to 2min12sec
* without unitchecks, its down to 1min50sec now
* it reduces the build time on a i7-4770 Windows tinderbox from 99min to
89min
* by now it also takes care of avoiding most string copying, although
this can certainly be considered overeager given that the file
creation on Windows took ~250ms per file before and still will take a
lot longer than any string operation with this change

Change-Id: I515432bdefe2b055c78b6ba97868adbde65d9165

0 files changed, 0 insertions, 0 deletions