init: Added --partial-clone-exclude option.

partial-clone-exclude option excludes projects during
partial clone. This is a comma-delimited project names
(from manifest.xml). This option is persisted and it
is used by the sync command.

A project that has been unparital'ed will remain unpartial if
that project's name is specified in the --partial-clone-exclude
option. The project name should match exactly.

Added
$ ./run_tests -v

Bug: [google internal] b/175712967
"I can't "unpartial" my androidx-main checkout"

$ rm -rf androidx-main/
$ mkdir androidx-main/
$ cd androidx-main/
$ repo_dev init -u https://android.googlesource.com/platform/manifest -b androidx-main --partial-clone --clone-filter=blob:limit=10M -m default.xml
$ repo_dev sync -c -j8

+ Verify a project is partial
$ cd frameworks/support/
$ git config -l | grep  'partial'

+ Unpartial a project.
$ /google/bin/releases/android/git_repack/git_unpartial

+ Verify project is unpartial
$ git config -l | grep  'partial'
$ cd ../..

+ Exclude the project from being unparial'ed after init and sync.
$ repo_dev init -u https://android.googlesource.com/platform/manifest -b androidx-main --partial-clone --clone-filter=blob:limit=10M --partial-clone-exclude="platform/frameworks/support,platform/frameworks/support-golden" -m default.xml

+ Verify project is unpartial
$ cd frameworks/support/
$ git config -l | grep  'partial'
$ cd ../..
$ repo_dev sync -c -j8
$ cd frameworks/support/
$ git config -l | grep  'partial'
$ cd ../..

+ Remove the project from exclude list and verify that project is partially cloned.
$ repo_dev init -u https://android.googlesource.com/platform/manifest -b androidx-main --partial-clone --clone-filter=blob:limit=10M --partial-clone-exclude= -m default.xml
$ repo_dev sync -c -j8
$ cd frameworks/support/
$ git config -l | grep  'partial'

Change-Id: Id5dba418eba1d3f54b54e826000406534c0ec196
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303162
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

manifest_xml.py

@@ -589,6 +589,12 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
       return self.manifestProject.config.GetString('repo.clonefilter')
     return None
 
+  @property
+  def PartialCloneExclude(self):
+    exclude = self.manifest.manifestProject.config.GetString(
+        'repo.partialcloneexclude') or ''
+    return set(x.strip() for x in exclude.split(','))
+
   @property
   def IsMirror(self):
     return self.manifestProject.config.GetBoolean('repo.mirror')