gitclone: We don't have to fetch, if revision is already present in local git repository

Because revision specifies worktree state exactly, and if we have it
already, we don't need to fetch anything -- it is possible to
checkout/reset to the revision right away.

The reason for this patch is that fetching is sometimes not very fast or
even slow. For example today, gitlab.com had some outage and I've
developed this patch while waiting for it to come back.

/reviewed-by @jerome, @kazuhiko
/reviewed-on !1

slapos/recipe/gitclone.py

@@ -34,7 +34,7 @@ import time
 import traceback
 
 from zc.buildout import UserError
-from subprocess import check_call, CalledProcessError
+from subprocess import call, check_call, CalledProcessError
 import subprocess
 
 try:
@@ -244,7 +244,13 @@ class Recipe(object):
       # first cleanup pyc files
       self.deletePycFiles(self.location)
 
-      # then update
+      # then update,
+      # but, to save time, only if we don't have the revision already
+      revision_already_fetched = \
+            self.revision and   \
+            call([self.git_command, 'rev-parse', '--verify', self.revision],
+                    cwd=self.location) == 0
+      if not revision_already_fetched:
         check_call([self.git_command, 'fetch', '--all'], cwd=self.location)
 
       # If develop parameter is set, don't reset/update.