Make `--patch` compatible with git diff patches

Filenames in git diff patches are prefixed by default by "a/" (source)
and "b/" destination. Such patches don't work with ruby-builds `--patch`
option since it internally uses `patch -p0`.

Prior workarounds were to use either an intermediate step:

    filterdiff --strip=1

or to generate the patch without the prefix in the first place:

    git diff --no-prefix ...

Now, git diff patches are detected by searching for this pattern:

    diff --git a/...

And `patch -p1` is used by default in such cases to strip the 1st
component of filename paths.

Fixes #521, closes #484

bin/ruby-build

@@ -846,9 +846,15 @@ isolated_gem_install() {
 }
 
 apply_ruby_patch() {
+  local patchfile
   case "$1" in
   ruby-* | jruby-* | rubinius-* )
-    patch -p0 --force -i "${2:--}"
+    patchfile="$(mktemp -t ruby-patch)"
+    cat "${2:--}" >"$patchfile"
+
+    local striplevel=0
+    grep -q '^diff --git a/' "$patchfile" && striplevel=1
+    patch -p$striplevel --force -i "$patchfile"
     ;;
   esac
 }

test/build.bats

@@ -85,9 +85,9 @@ OUT
   stub brew false
   stub_make_install
   stub_make_install
-  stub patch ' : echo patch "$@" >> build.log'
+  stub patch ' : echo patch "$@" | sed -E "s/\.[[:alnum:]]+$/.XXX/" >> build.log'
 
-  install_fixture --patch definitions/needs-yaml
+  echo | TMPDIR="$TMP" install_fixture --patch definitions/needs-yaml
   assert_success
 
   unstub make
@@ -97,7 +97,33 @@ OUT
 yaml-0.1.6: --prefix=$INSTALL_ROOT
 make -j 2
 make install
-patch -p0 --force -i -
+patch -p0 --force -i $TMP/ruby-patch.XXX
+ruby-2.0.0: --prefix=$INSTALL_ROOT
+make -j 2
+make install
+OUT
+}
+
+@test "apply ruby patch from git diff before building" {
+  cached_tarball "yaml-0.1.6"
+  cached_tarball "ruby-2.0.0"
+
+  stub brew false
+  stub_make_install
+  stub_make_install
+  stub patch ' : echo patch "$@" | sed -E "s/\.[[:alnum:]]+$/.XXX/" >> build.log'
+
+  echo 'diff --git a/script.rb' | TMPDIR="$TMP" install_fixture --patch definitions/needs-yaml
+  assert_success
+
+  unstub make
+  unstub patch
+
+  assert_build_log <<OUT
+yaml-0.1.6: --prefix=$INSTALL_ROOT
+make -j 2
+make install
+patch -p1 --force -i $TMP/ruby-patch.XXX
 ruby-2.0.0: --prefix=$INSTALL_ROOT
 make -j 2
 make install