Lengthen lastchange VERSION string to show full hash and commit position.

Now that the DMServer has been fixed to accept 'agent' strings longer than
64 characters (http://crbug.com/406948), we can re-lengthen the string
produced by lastchange.py to include both the full git hash and the
Cr-Commit-Position.

R=joaodasilva@chromium.org, scottmg@chromium.org
BUG=406783

Review URL: https://codereview.chromium.org/527103004

Cr-Commit-Position: refs/heads/master@{#292963}

build/util/lastchange.py

@@ -99,8 +99,6 @@ def FetchGitRevision(directory):
   Returns:
     A VersionInfo object or None on error.
   """
-  # TODO(agable): Re-add the commit position after the lastchange value can
-  # accept strings longer than 64 characters. See crbug.com/406783.
   hsh = ''
   proc = RunGitCommand(directory, ['rev-parse', 'HEAD'])
   if proc:
@@ -109,9 +107,17 @@ def FetchGitRevision(directory):
       hsh = output
   if not hsh:
     return None
-  # TODO(agable): Figure out a way to use the full hash instead of just a
-  # 12-character short hash. See crbug.com/406783.
-  return VersionInfo('git', hsh[:12])
+  pos = ''
+  proc = RunGitCommand(directory, ['show', '-s', '--format=%B', 'HEAD'])
+  if proc:
+    output = proc.communicate()[0]
+    if proc.returncode == 0 and output:
+      for line in reversed(output.splitlines()):
+        if line.startswith('Cr-Commit-Position:'):
+          pos = line.rsplit()[-1].strip()
+  if not pos:
+    return VersionInfo('git', hsh)
+  return VersionInfo('git', '%s-%s' % (hsh, pos))
 
 
 def FetchGitSVNURLAndRevision(directory, svn_url_regex):