fetchpatch: add "decode" argument for gerrit

(and gitiles)

This allows fetching a patch from servers that return them
base64-encoded, like this:

    fetchpatch {
      name = "gcc.patch";
      url = "f37ae3b1a8^!?format=TEXT";
      decode = "base64 -d";
      sha256 = "11j1bqz2p8xrfzgfrylgdvmqs45489c4ckl7l0ra1dpfgbqy94a8";
    }

pkgs/build-support/fetchpatch/default.nix

@@ -8,6 +8,7 @@
 
 { relative ? null
 , stripLen ? 0
+, decode ? "cat" # custom command to decode patch e.g. base64 -d
 , extraPrefix ? null
 , excludes ? []
 , includes ? []
@@ -36,6 +37,17 @@ fetchurl ({
       exit 1
     fi
 
+    set +e
+    ${decode} < "$out" > "$tmpfile"
+    if [ $? -ne 0 ] || [ ! -s "$tmpfile" ]; then
+        echo 'Failed to decode patch with command "'${lib.escapeShellArg decode}'"' >&2
+        echo 'Fetched file was (limited to 128 bytes):' >&2
+        od -A x -t x1z -v -N 128 "$out" >&2
+        exit 1
+    fi
+    set -e
+    mv "$tmpfile" "$out"
+
     "${patchutils}/bin/lsdiff" \
       ${lib.optionalString (relative != null) "-p1 -i ${lib.escapeShellArg relative}/'*'"} \
       "$out" \
@@ -76,5 +88,6 @@ fetchurl ({
     mv "$tmpfile" "$out"
   '' + postFetch;
 } // builtins.removeAttrs args [
-  "relative" "stripLen" "extraPrefix" "excludes" "includes" "revert" "postFetch"
+  "relative" "stripLen" "decode" "extraPrefix" "excludes" "includes" "revert"
+  "postFetch"
 ])