data: fix gimp-release file on Windows.

The '\n' newline is transformed into a space (0x20) when building
natively on Windows. And unfortunately g_key_file_load_from_file() does
not like this.

The autotools rule works properly on all platforms according to my
tests.

Note that at first, I thought it was a LF vs. CRLF issue, and tried to
special-case Windows, but I realized it's just that 'echo' command
seemingly transform newlines unexpectedly in its Windows form. I didn't
want to waste too much time experimenting alternative calls, especially
as I have to test through CI (so each test is very costly, time-wise).
Instead I go the alternative way of using a pre-generated conf file.

data/gimp-release.in

@@ -0,0 +1,2 @@
+[package]
+revision=@REVISION@

data/meson.build

@@ -15,12 +15,14 @@ meson.add_install_script('sh', '-c',
   'mkdir -p $MESON_INSTALL_DESTDIR_PREFIX/@0@'.format(gimpdatadir / 'fonts')
 )
 
-custom_target('gimp-release',
-  output: [ 'gimp-release' ],
-  command: [
-    'echo', ['[package]' + '\nrevision=@0@'.format(get_option('revision'))]
-  ],
-  capture: true,
+release_conf = configuration_data()
+release_conf.set('REVISION', get_option('revision'))
+
+gimp_release = 'gimp-release'
+configure_file(
+  input : gimp_release + '.in',
+  output: gimp_release,
+  configuration: release_conf,
   install: true,
   install_dir: gimpdatadir,
 )