syncthing: Scan on absolute path creates incorrect absolute duplicate file infos

Reproducer

Run scan through rest API on a “virtually” absolute path that has no changed children, e.g. in my case folder root at /home/simon/test-sync and

curl -X POST -H "X-API-KEY: myapikey" "localhost:8385/rest/db/scan?folder=3vbey-elvwr&sub=/foo"

Result:
State counter increases and scanner debug log shows the problem:

[BXAEQ] 2018/03/10 11:09:21.303458 walk.go:101: DEBUG: Walk [/foo] 131072 Matcher/[nonreach* **/nonreach* nonreach*/** **/nonreach*/**]@0xc4202522d0
[BXAEQ] 2018/03/10 11:09:21.303837 walk.go:316: DEBUG: to hash: /foo File{Name:"/foo", Sequence:0, Permissions:0664, ModTime:2017-12-27 23:10:28.912229075 +0100 CET, Version:{[{BXAEQNX 1}]}, Length:741, Deleted:false, Invalid:false, NoPermissions:true, Blocks:[]}
[BXAEQ] 2018/03/10 11:09:21.303876 walk.go:186: DEBUG: real to hash: /foo
[BXAEQ] 2018/03/10 11:09:21.304035 walk.go:164: DEBUG: Walk progress done 3vbey-elvwr [/foo] 131072 Matcher/[nonreach* **/nonreach* nonreach*/** **/nonreach*/**]@0xc4202522d0

Expected:
No change at all.

Proposed fix

According to this comment the absolute path is valid, so it shouldn’t throw an error, but scan normally removing the leading path separator.