Reland "bot_update: Set user.{name,email} when applying patch refs."

This is a reland of e86fe982

Original change's description:
> bot_update: Set user.{name,email} when applying patch refs.
> 
> git rebase complains when user name/email has not been configured, as was
> the case for the Chromium trybot.
> 
> Bug: 643346
> Change-Id: Iab7dac0a98608e09b63a53d5bacb98925701fbfd
> Reviewed-on: https://chromium-review.googlesource.com/1031524
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Bug: 643346
Change-Id: I4b4d07675222d39d704560ed84f9c221e87aba8b
Reviewed-on: https://chromium-review.googlesource.com/1036309Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -7,6 +7,7 @@
 
 import cStringIO
 import codecs
+from contextlib import contextmanager
 import copy
 import ctypes
 import json
@@ -331,6 +332,24 @@ def gclient_configure(solutions, target_os, target_os_only, target_cpu,
         solutions, target_os, target_os_only, target_cpu, git_cache_dir))
 
 
+@contextmanager
+def git_config_if_not_set(key, value):
+  """Set git config for key equal to value if key was not set.
+
+  If key was not set, unset it once we're done."""
+  should_unset = True
+  try:
+    git('config', '--global', key)
+    should_unset = False
+  except SubprocessFailed as e:
+    git('config', '--global', key, value)
+  try:
+    yield
+  finally:
+    if should_unset:
+      git('config', '--global', '--unset', key)
+
+
 def gclient_sync(
     with_branch_heads, with_tags, shallow, revisions, break_repo_locks,
     disable_syntax_validation, gerrit_repo, gerrit_ref, gerrit_reset,
@@ -903,21 +922,24 @@ def ensure_checkout(solutions, revisions, first_sln, target_os, target_os_only,
   # This forces gclient to always treat solutions deps as unmanaged.
   for solution_name in list(solution_dirs):
     gc_revisions[solution_name] = 'unmanaged'
-  # Let gclient do the DEPS syncing.
-  # The branch-head refspec is a special case because its possible Chrome
-  # src, which contains the branch-head refspecs, is DEPSed in.
-  gclient_output = gclient_sync(
-      BRANCH_HEADS_REFSPEC in refs,
-      TAGS_REFSPEC in refs,
-      shallow,
-      gc_revisions,
-      break_repo_locks,
-      disable_syntax_validation,
-      gerrit_repo,
-      gerrit_ref,
-      gerrit_reset,
-      gerrit_rebase_patch_ref,
-      apply_patch_on_gclient)
+
+  with git_config_if_not_set('user.name', 'chrome-bot'):
+    with git_config_if_not_set('user.email', 'chrome-bot@chromium.org'):
+      # Let gclient do the DEPS syncing.
+      # The branch-head refspec is a special case because its possible Chrome
+      # src, which contains the branch-head refspecs, is DEPSed in.
+      gclient_output = gclient_sync(
+          BRANCH_HEADS_REFSPEC in refs,
+          TAGS_REFSPEC in refs,
+          shallow,
+          gc_revisions,
+          break_repo_locks,
+          disable_syntax_validation,
+          gerrit_repo,
+          gerrit_ref,
+          gerrit_reset,
+          gerrit_rebase_patch_ref,
+          apply_patch_on_gclient)
 
   # Now that gclient_sync has finished, we should revert any .DEPS.git so that
   # presubmit doesn't complain about it being modified.