Update Buck to recent version

This version fixes this bug [1] that allows us to eliminate these ugly
shell backticks and switch again to escaped macro like invocations:

  \$(dirname $OUT)

[1] https://github.com/facebook/buck/issues/212

Change-Id: Ie43b3ff6dfc89c4d840f2ff75c64ac0a5c6346b6

.buckversion

@@ -1 +1 @@
-f0759b0bfff1368bc715371c383dfd8c873f9991
+c38b9899f7952382963cb7822bbdfc9104b42e70

gerrit-gwtui/BUCK

@@ -15,7 +15,7 @@ genrule(
     ' gerrit_ui/gerrit_ui.nocache.js' +
     ' gerrit_ui/dbg_gerrit_ui.nocache.js;' +
     'unzip -qo $(location :ui_opt);' +
-    'mkdir -p `dirname $OUT`;' +
+    'mkdir -p \$(dirname $OUT);' +
     'zip -qr $OUT .',
   deps = [
     ':ui_dbg',

gerrit-gwtui/gwt.defs

@@ -56,7 +56,7 @@ def gwt_user_agent_permutations(
     genrule(
       name = '%s_gwtxml_gen' % gwt_name,
       cmd = 'cd $TMP;' +
-        ('mkdir -p `dirname %s`;' % gwt) +
+        ('mkdir -p \$(dirname %s);' % gwt) +
         ('echo "%s">%s;' % (xml, gwt)) +
         'zip -qr $OUT .',
       out = jar,

tools/default.defs

@@ -80,7 +80,7 @@ def gerrit_plugin(
     type = 'plugin',
     visibility = ['PUBLIC']):
   from multiprocessing import cpu_count
-  mf_cmd = 'v=`git describe HEAD`;'
+  mf_cmd = 'v=\$(git describe HEAD);'
   if manifest_file:
     mf_src = [manifest_file]
     mf_cmd += 'sed "s:@VERSION@:$v:g" $SRCS >$OUT'