Add remote alias support in manifest

The `alias` is an optional attribute in element `remote`. It can be
used to override attibute `name` to be set as the remote name in each
project's .git/config. Its value can be duplicated while attribute
`name` has to be unique across the manifest file. This helps each
project to be able to have same remote name which actually points
to different remote url.

It eases some automation scripts to be able to checkout/push to same
remote name but actually different remote url, like:

repo forall -c "git checkout -b work same_remote/work"
repo forall -c "git push same_remote work:work"

for example:
The manifest with 'alias' will look like:

<?xml version='1.0' encoding='UTF-8'?>
<manifest>
  <remote alias="same_alias" fetch="git://git.external1.org/" name="ext1"
      review="http://review.external1.org"/>
  <remote alias="same_alias" fetch="git://git.external2.org/" name="ext2"
      review="http://review.external2.org"/>
  <remote alias="same_alias" fetch="ssh://git.internal.com:29418" name="int"
      review="http://review.internal.com"/>
  <default remote="int" revision="int-branch" sync-j="2"/>
  <project name="path/to/project1" path="project1" remote="ext1"/>
  <project name="path/to/project2" path="project2" remote="ext2"/>
  <project name="path/to/project3" path="project3"/>
  ...
</manifest>

In each project, use command "git remote -v"

project1:
same_alias  git://git.external1.org/project1 (fetch)
same_alias  git://git.external1.org/project1 (push)

project2:
same_alias  git://git.external2.org/project2 (fetch)
same_alias  git://git.external2.org/project2 (push)

project3:
same_alias  ssh://git.internal.com:29418/project3 (fetch)
same_alias  ssh://git.internal.com:29418/project3 (push)

Change-Id: I2c48263097ff107f0c978f3e83966ae71d06cb90

docs/manifest-format.txt

@@ -32,6 +32,7 @@ following DTD:
   
     <!ELEMENT remote (EMPTY)>
     <!ATTLIST remote name         ID    #REQUIRED>
+    <!ATTLIST remote alias        CDATA #IMPLIED>
     <!ATTLIST remote fetch        CDATA #REQUIRED>
     <!ATTLIST remote review       CDATA #IMPLIED>
   
@@ -89,6 +90,12 @@ name specified here is used as the remote name in each project's
 .git/config, and is therefore automatically available to commands
 like `git fetch`, `git remote`, `git pull` and `git push`.
 
+Attribute `alias`: The alias, if specified, is used to override
+`name` to be set as the remote name in each project's .git/config.
+Its value can be duplicated while attribute `name` has to be unique
+in the manifest file. This helps each project to be able to have
+same remote name which actually points to different remote url.
+
 Attribute `fetch`: The Git URL prefix for all projects which use
 this remote.  Each project's name is appended to this prefix to
 form the actual URL used to clone the project.

manifest_xml.py

@@ -41,12 +41,14 @@ class _Default(object):
 class _XmlRemote(object):
   def __init__(self,
                name,
+               alias=None,
                fetch=None,
                manifestUrl=None,
                review=None):
     self.name = name
     self.fetchUrl = fetch
     self.manifestUrl = manifestUrl
+    self.remoteAlias = alias
     self.reviewUrl = review
     self.resolvedFetchUrl = self._resolveFetchUrl()
 
@@ -62,7 +64,10 @@ class _XmlRemote(object):
 
   def ToRemoteSpec(self, projectName):
     url = self.resolvedFetchUrl.rstrip('/') + '/' + projectName
-    return RemoteSpec(self.name, url, self.reviewUrl)
+    remoteName = self.name
+    if self.remoteAlias:
+      remoteName = self.remoteAlias
+    return RemoteSpec(remoteName, url, self.reviewUrl)
 
 class XmlManifest(object):
   """manages the repo configuration file"""
@@ -451,12 +456,15 @@ class XmlManifest(object):
     reads a <remote> element from the manifest file
     """
     name = self._reqatt(node, 'name')
+    alias = node.getAttribute('alias')
+    if alias == '':
+      alias = None
     fetch = self._reqatt(node, 'fetch')
     review = node.getAttribute('review')
     if review == '':
       review = None
     manifestUrl = self.manifestProject.config.GetString('remote.origin.url')
-    return _XmlRemote(name, fetch, manifestUrl, review)
+    return _XmlRemote(name, alias, fetch, manifestUrl, review)
 
   def _ParseDefault(self, node):
     """