Relative paths of files() are passed to a command when `custom_target` is used

I tried to figure out why a switch from run_target to custom_target caused ctags to generate tags containing relative paths. I found https://github.com/mesonbuild/meson/pull/1011 where run_target had been fixed, but it seems that custom_target hasn't been changed similarly yet.

$ cat meson.build
project('test', 'c')

all_files = files('main.c')

custom_target(
	'custom',
	output : 'custom',
	command : ['echo'] + all_files)

run_target(
	'run',
	command : ['echo'] + all_files)

$ touch main.c

$ ../meson/meson.py build
The Meson build system
Version: 0.47.0.dev1
Source dir: /home/ubuntu/TEST
Build dir: /home/ubuntu/TEST/build
Build type: native build
Project name: test
Native C compiler: cc (gcc 5.4.0 "cc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609")
Build machine cpu family: x86_64
Build machine cpu: x86_64
Build targets in project: 2
Found ninja-1.7.2 at /usr/bin/ninja

$ ninja -C ./build/ custom
ninja: Entering directory `./build/'
[1/1] Generating custom with a custom command.
../main.c

$ ninja -C ./build/ run
ninja: Entering directory `./build/'
[0/1] Running external command run.
/home/ubuntu/TEST/build/../main.c

Reproduce the example from meson.build, then run the custom and run targets with Ninja and compare the emitted paths. Read the existing handling for run_target and trace the corresponding custom_target path; done means both targets handle files('main.c') consistently and the behavior is covered by an appropriate regression check.