Enable remotes to define their own revision

Some projects use multiple remotes.
In some cases these remotes have different naming conventions.
Add an option to define a revision in the remote configuration.

The `project` revision takes precedence over `remote` and `default`.
The `remote` revision takes precedence over `default`.
The `default` revision acts as a fall back as it originally did.

Change-Id: I2b376160d45d48b0bab840c02a3eef1a1e32cf6d
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index e48b75f..f187bfa 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -35,6 +35,7 @@
     <!ATTLIST remote alias        CDATA #IMPLIED>
     <!ATTLIST remote fetch        CDATA #REQUIRED>
     <!ATTLIST remote review       CDATA #IMPLIED>
+    <!ATTLIST remote revision     CDATA #IMPLIED>
 
     <!ELEMENT default (EMPTY)>
     <!ATTLIST default remote      IDREF #IMPLIED>
@@ -112,6 +113,10 @@
 are uploaded to by `repo upload`.  This attribute is optional;
 if not specified then `repo upload` will not function.
 
+Attribute `revision`: Name of a Git branch (e.g. `master` or
+`refs/heads/master`). Remotes with their own revision will override
+the default revision.
+
 Element default
 ---------------
 
@@ -208,7 +213,8 @@
 (e.g. just "master") or absolute (e.g. "refs/heads/master").
 Tags and/or explicit SHA-1s should work in theory, but have not
 been extensively tested.  If not supplied the revision given by
-the default element is used.
+the remote element is used if applicable, else the default
+element is used.
 
 Attribute `dest-branch`: Name of a Git branch (e.g. `master`).
 When using `repo upload`, changes will be submitted for code