Add total count and iteration count to forall environment

For long-running forall commands sometimes it's useful to know which
iteration is currently running. Add REPO_I and REPO_COUNT environment
variables to reflect the current iteration count as well as the total
number of iterations so that the user can build simple status
indicators.

Example:

    $ repo forall -c 'echo $REPO_I / $REPO_COUNT; git gc'
    1 / 579
    Counting objects: 41, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (19/19), done.
    Writing objects: 100% (41/41), done.
    Total 41 (delta 21), reused 41 (delta 21)
    2 / 579
    Counting objects: 53410, done.
    Delta compression using up to 8 threads.
    Compressing objects: 100% (10423/10423), done.
    Writing objects: 100% (53410/53410), done.
    Total 53410 (delta 42513), reused 53410 (delta 42513)
    3 / 579
    ...

Change-Id: I9f28b0d8b7debe423eed3b4bc1198b23e40c0c50
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>

subcmds/forall.py

@@ -87,6 +87,12 @@ revision to a locally executed git command, use REPO_LREV.
 REPO_RREV is the name of the revision from the manifest, exactly
 as written in the manifest.
 
+REPO_COUNT is the total number of projects being iterated.
+
+REPO_I is the current (1-based) iteration count. Can be used in
+conjunction with REPO_COUNT to add a simple progress indicator to your
+command.
+
 REPO__* are any extra environment variables, specified by the
 "annotation" element under any project element.  This can be useful
 for differentiating trees based on user-specific criteria, or simply
@@ -178,7 +184,9 @@ without iterating through the remaining projects.
     else:
       projects = self.FindProjects(args)
 
-    for project in projects:
+    os.environ['REPO_COUNT'] = str(len(projects))
+
+    for (cnt, project) in enumerate(projects):
       env = os.environ.copy()
       def setenv(name, val):
         if val is None:
@@ -190,6 +198,7 @@ without iterating through the remaining projects.
       setenv('REPO_REMOTE', project.remote.name)
       setenv('REPO_LREV', project.GetRevisionId())
       setenv('REPO_RREV', project.revisionExpr)
+      setenv('REPO_I', str(cnt + 1))
       for a in project.annotations:
         setenv("REPO__%s" % (a.name), a.value)