Add overwrite branch option to commit files header

Will be used to allow the API to force push a new branch over the top
of an existing branch.

See gitlab-ce#45035
2 jobs for jr-force-push in 1 minute and 20 seconds
latest
Status Job ID Name Coverage
  Test
passed #103978222
build

00:01:20

failed #103978223
check-grpc-proto-clients

00:00:59

 
Name Stage Failure
failed
check-grpc-proto-clients Test
+	0x49, 0x02, 0xc7, 0x77, 0xe0, 0x24, 0x81, 0x19, 0x1b, 0xa3, 0x24, 0x70, 0xe4, 0x9e, 0x9c, 0xa0,
+ 0x69, 0xf7, 0xb4, 0xed, 0x29, 0xed, 0x9e, 0x3e, 0x94, 0x47, 0x51, 0xe2, 0x28, 0x48, 0x9a, 0x49,
+ 0x4c, 0xd0, 0xa4, 0x99, 0xe4, 0xe4, 0xb0, 0x72, 0x8f, 0x4c, 0xf5, 0x37, 0xc5, 0x2b, 0x7f, 0x06,
+ 0x00, 0x00, 0xff, 0xff, 0x3a, 0xa1, 0x31, 0x14, 0xd0, 0x18, 0x00, 0x00,
}
error: detected changes
Makefile:41: recipe for target 'check-grpc-proto-clients' failed
make: *** [check-grpc-proto-clients] Error 1
ERROR: Job failed: exit code 1