rust-format: pin the formatted to a specific nightly version

Version 1.50 change the way rust-format behave. We pin the version for now, we
can figure out something better later.

tests/hghave.py

@@ -1058,11 +1058,12 b' def has_pytype():'
     return version and sv(_bytes2sys(version.group(0))) >= sv('2019.10.17')
 
 
-@check("rustfmt", "rustfmt tool")
+@check("rustfmt", "rustfmt tool at version nightly-2020-10-04")
 def has_rustfmt():
     # We use Nightly's rustfmt due to current unstable config options.
     return matchoutput(
-        '`rustup which --toolchain nightly rustfmt` --version', b'rustfmt'
+        '`rustup which --toolchain nightly-2020-10-04 rustfmt` --version',
+        b'rustfmt',
     )
 
 

tests/test-check-rust-format.t

@@ -3,7 +3,7 b''
   $ . "$TESTDIR/helpers-testrepo.sh"
 
   $ cd "$TESTDIR"/..
-  $ RUSTFMT=$(rustup which --toolchain nightly rustfmt)
+  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
   $ for f in `testrepohg files 'glob:**/*.rs'` ; do
   >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
   > done