Add polygerrit.war target to build only PolyGerrit UI

Developers working on the PolyGerrit UI shouldn't have to wait an
extra 30-60s to build the GWT UI into the binary.

Change-Id: If2958a9b09d9a998cf2ab00b0ee60e62d8dee564

.buckconfig

@@ -9,6 +9,7 @@
   firefox = //:firefox
   gerrit = //:gerrit
   headless = //:headless
+  polygerrit = //:polygerrit
   release = //:release
   safari = //:safari
   soyc = //gerrit-gwtui:ui_soyc

BUCK

@@ -5,6 +5,7 @@ gerrit_war(name = 'headless', ui = None)
 gerrit_war(name = 'chrome',      ui = 'ui_chrome')
 gerrit_war(name = 'firefox',     ui = 'ui_firefox')
 gerrit_war(name = 'safari',      ui = 'ui_safari')
+gerrit_war(name = 'polygerrit',  ui = 'polygerrit')
 gerrit_war(name = 'withdocs', docs = True)
 gerrit_war(name = 'release',  ui = 'ui_optdbg_r', docs = True, context = ['//plugins:core'],  visibility = ['//tools/maven:'])
 

tools/build.defs

@@ -61,18 +61,19 @@ def war(
   )
 
 def gerrit_war(name, ui = 'ui_optdbg', context = [], docs = False, visibility = []):
+  ui_deps = []
+  if ui:
+    ui_deps.append('//polygerrit-ui/app:polygerrit_ui')
+    if ui != 'polygerrit':
+      ui_deps.append('//gerrit-gwtui:%s' % ui)
   war(
     name = name,
     libs = LIBS + ['//gerrit-war:version'],
     pgmlibs = PGMLIBS,
-    context = [
+    context = ui_deps + context + [
       '//gerrit-main:main_bin',
       '//gerrit-war:webapp_assets',
-    ] + ([
-      '//gerrit-gwtui:' + ui,
-      '//polygerrit-ui/app:polygerrit_ui',
-    ] if ui else []) +
-    context,
+    ],
     docs = docs,
     visibility = visibility,
   )