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 = "https://chromium.googlesource.com/aosp/platform/external/libchrome/+/f37ae3b1a873d74182a2ac31d96742ead9c1f523^!?format=TEXT";
decode = "base64 -d";
sha256 = "11j1bqz2p8xrfzgfrylgdvmqs45489c4ckl7l0ra1dpfgbqy94a8";
}