add tools/patch_from_svn to ease patching from svn diffs

Change-Id: Ib8144514b7d73505c5ab1faa30b53d606463e20f

tools/patch_from_svn

+#!/bin/bash
+if [ -z "$1" ]; then
+    echo "Usage: patch_from_svn some.patch"
+    echo "The patch should be generated by 'svn diff -c <REV>'"
+    exit 1
+fi
+cat $1 | sed -e 's/src\/baidu\/rpc\//brpc\//g' > $1.brpc_os
+EXTRA_ARGS=
+if [ -z "$DO_RUN" ]; then
+    EXTRA_ARGS="--dry-run $EXTRA_ARGS"
+    echo "*** This is a dry-run. To really apply, run: DO_RUN=1 tools/patch_from_svn $1"
+fi
+patch -p0 -u $EXTRA_ARGS < $1.brpc_os