fetchpatch: add addPrefixes argument

Sometimes patches start without a leading prefix. We default to strip
one prefix or path component from patches (-p1) in the patchPhase in
stdenv.

As all patches should therefore be in this format, fetchpatch should
have an option to normalize patch paths. This commit introduces a new
argument to fetchpatch called addPrefixes that adds one patch prefix to
the old and new paths in a patch before putting it into the store.

pkgs/build-support/fetchpatch/default.nix

@@ -4,8 +4,8 @@
 # often change with updating of git or cgit.
 # stripLen acts as the -p parameter when applying a patch.
 
-{ fetchurl, patchutils }:
-{ stripLen ? 0, ... }@args:
+{ lib, fetchurl, patchutils }:
+{ stripLen ? 0, addPrefixes ? false, ... }@args:
 
 fetchurl ({
   postFetch = ''
@@ -16,8 +16,12 @@ fetchurl ({
         "${patchutils}/bin/filterdiff" \
         --include={} \
         --strip=${toString stripLen} \
+        ${lib.optionalString addPrefixes ''
+           --addoldprefix=a/ \
+           --addnewprefix=b/ \
+        ''} \
         --clean "$out" > "$tmpfile"
     mv "$tmpfile" "$out"
     ${args.postFetch or ""}
   '';
-} // builtins.removeAttrs args ["stripLen"])
+} // builtins.removeAttrs args ["stripLen" "addPrefixes"])