Revert of scm.GIT.IsValidRevision: Only return True if the given object is a commit

Revert of scm.GIT.IsValidRevision: Only return True if the given object is a commit (https://codereview.chromium.org/329823002/)

Reason for revert:
Speculative revert to see if this is the cause of auto_roll_test.py to fail in build/ presubmit.

Original issue's description:
> scm.GIT.IsValidRevision: Only return True if the given object is a commit
> 
> BUG=383476
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=276487

TBR=iannucci@chromium.org,maruel@chromium.org,borenet@google.com
NOTREECHECKS=true
NOTRY=true
BUG=383476

[Ryan Tseng, 2014-06-11 22:10:29]

Created Revert of scm.GIT.IsValidRevision: Only return True if the given object
is a commit

[commit-bot: I haz the power, 2014-06-11 22:10:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hinoka@google.com/329393002/1

[M-A Ruel, 2014-06-11 22:12:23]

lgtm

[commit-bot: I haz the power, 2014-06-11 22:27:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-11 22:27:35]

Failed to commit the patch.
Sending        depot_tools/scm.py
Sending        depot_tools/tests/scm_unittest.py
Transmitting file data ..

[Ryan Tseng, 2014-06-11 23:08:44]

The CQ bit was checked by hinoka@google.com

[commit-bot: I haz the power, 2014-06-11 23:09:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hinoka@google.com/329393002/1

[commit-bot: I haz the power, 2014-06-11 23:09:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-11 23:09:21]

Failed to apply patch for depot_tools/scm.py:
While running patch -p1 --forward --force --no-backup-if-mismatch;
  patching file depot_tools/scm.py
  Hunk #1 FAILED at 491.
  1 out of 1 hunk FAILED -- saving rejects to file depot_tools/scm.py.rej

Patch:       depot_tools/scm.py
Index: scm.py
diff --git depot_tools/scm.py depot_tools/scm.py
index
d60453dceff9c1e1362d0be94e368e2112619663..eb5c52403c3e2595d14b133a7d0d00e289811d10
100644
--- a/depot_tools/scm.py
+++ b/depot_tools/scm.py
@@ -491,15 +491,12 @@
       sha = GIT.Capture(['rev-parse', lookup_rev], cwd=cwd).lower()
       if lookup_rev != rev:
         # Make sure we get the original 40 chars back.
-        if rev.lower() != sha:
-          return False
+        return rev.lower() == sha
       if sha_only:
-        if not sha.startswith(rev.lower()):
-          return False
+        return sha.startswith(rev.lower())
+      return True
     except subprocess2.CalledProcessError:
       return False
-    obj_type = GIT.Capture(['cat-file', '-t', rev], cwd=cwd).strip()
-    return obj_type == 'commit'
 
   @classmethod
   def AssertVersion(cls, min_version):