Add sync-c option to manifest

There are use-cases when fetching all branch is impractical and
we really need to fetch only one branch/tag.
e.g. there is a large project with binaries and every update of a
binary file is put to a separate branch.
The whole project history might be too large to allow users fetch it.

Add 'sync-c' option to 'project' and 'default' tags to make it possible
to configure 'sync-c' behavior at per-project and per-manifest level.

Note that currently there is no possibility to revert boolean flag from
command line. If 'sync-c' is set in manifest then you cannot make
full fetch by providing a repo tool argument.

Change-Id: Ie36fe5737304930493740370239403986590f593
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index 764e41e..5378965 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -39,6 +39,7 @@
     <!ATTLIST default remote   IDREF #IMPLIED>
     <!ATTLIST default revision CDATA #IMPLIED>
     <!ATTLIST default sync-j   CDATA #IMPLIED>
+    <!ATTLIST default sync-c   CDATA #IMPLIED>
 
     <!ELEMENT manifest-server (EMPTY)>
     <!ATTLIST url              CDATA #REQUIRED>
@@ -49,6 +50,7 @@
     <!ATTLIST project remote   IDREF #IMPLIED>
     <!ATTLIST project revision CDATA #IMPLIED>
     <!ATTLIST project groups   CDATA #IMPLIED>
+    <!ATTLIST project sync-c   CDATA #IMPLIED>
 
     <!ELEMENT annotation (EMPTY)>
     <!ATTLIST annotation name  CDATA #REQUIRED>