looks like a CL cherry pick is missing from the 4.19 branch ?  out of tree builds are failing w/4.19 because tools are writing to write to the source repo.

  make[1]: Entering directory '/build/cheza/var/cache/portage/sys-kernel/chromeos-kernel-4_19'
   GEN     ./Makefile
  * ACCESS DENIED:  open_wr:      /mnt/host/source/src/third_party/kernel/v4.19/.git/index.lock

 make[1]: Leaving directory '/build/cheza/var/cache/portage/sys-kernel/chromeos-kernel-4_19'
 >>> Source compiled.
  * --------------------------- ACCESS VIOLATION SUMMARY ---------------------------
  * LOG FILE: "/var/log/sandbox/sandbox-27958.log"
  * 
 VERSION 1.0
 FORMAT: F - Function called
 FORMAT: S - Access Status
 FORMAT: P - Path as passed to function
 FORMAT: A - Absolute Path (not canonical)
 FORMAT: R - Canonical Path
 FORMAT: C - Command Line
 
 F: open_wr
 S: deny
 P: /mnt/host/source/src/third_party/kernel/v4.19/.git/index.lock
 A: /mnt/host/source/src/third_party/kernel/v4.19/.git/index.lock
 R: /mnt/host/source/src/third_party/kernel/v4.19/.git/index.lock
 C: git status -uno --porcelain 
  * --------------------------------------------------------------------------------

can test with the CL here:
  https://chromium-review.googlesource.com/1316147