depot_tools: Don't use carets when checking if a revision exists.

On Windows, git can be either an executable or batch script, each of
which requires carets (^) to be escaped in different ways.

This makes it hard to use `git rev-parse --verify REV^{commit}`, so we
truncate full hash revisions to achieve the same effect instead.

Change-Id: I01578620706a1bab75a2decc55d9b4f1ac3afe28
Bug: 1065307
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2127968
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

scm.py

@@ -378,14 +378,16 @@ class GIT(object):
 
   @staticmethod
   def ResolveCommit(cwd, rev):
-    if sys.platform.startswith('win'):
-      # Windows .bat scripts use ^ as escape sequence, which means we have to
-      # escape it with itself for every .bat invocation.
-      needle = '%s^^{commit}' % rev
-    else:
-      needle = '%s^{commit}' % rev
+    # We do this instead of rev-parse --verify rev^{commit}, since on Windows
+    # git can be either an executable or batch script, each of which requires
+    # escaping the caret (^) a different way.
+    if gclient_utils.IsFullGitSha(rev):
+      # git-rev parse --verify FULL_GIT_SHA always succeeds, even if we don't
+      # have FULL_GIT_SHA locally. Removing the last character forces git to
+      # check if FULL_GIT_SHA refers to an object in the local database.
+      rev = rev[:-1]
     try:
-      return GIT.Capture(['rev-parse', '--quiet', '--verify', needle], cwd=cwd)
+      return GIT.Capture(['rev-parse', '--quiet', '--verify', rev], cwd=cwd)
     except subprocess2.CalledProcessError:
       return None