sync: respect sync-c manifest option

The documentation states that a `sync-c` attribute in the manifest file
can set a default for whether only the current branch should be fetched
or all branches. This seems to have been broken for some time.

Commit 7356114 introduced the `--no-current-branch` CLI option and
relied on getting `None` via `optparse` if neither `--current-branch`
nor `--no-current-branch` was set to distinguish it from a boolean
value. If `None` was received, it would read the value from the manifest
option `sync-c`. The parsing went through the utility function
`_GetCurrentBranchOnly` which returned `True` if `--current-branch` had
been given on the command-line, or fell back on the "superproject"
setting, which would either return `True` or `None`. This would
incorrectly make `repo` fall back to the manifest setting even if the
user had given `--no-current-branch` if no superproject was requested --
the manifest became "too powerful":

Command-line         Using superproject  → `current_branch_only`
------------         ------------------  -----------------------
                     No                  From manifest
                     Yes                 True
--current-branch     No                  True
--current-branch     Yes                 True
--no-current-branch  No                  From manifest ← wrong
--no-current-branch  Yes                 True

In commit 0cb6e92 the superproject configuration value reading changed
from something that could return `None` to something that always
returned a boolean. If it returned `False`, this would then incorrectly
make `repo` ignore the manifest option even if neither
`--current-branch` nor `--no-current-branch` had been given. The
manifest default became useless:

Command-line         Using superproject  → `current_branch_only`
------------         ------------------  -----------------------
                     No                  False ← wrong
                     Yes                 True
--current-branch     No                  True
--current-branch     Yes                 True
--no-current-branch  No                  False
--no-current-branch  Yes                 True

By swapping the order in which the command-line option target and the
superproject setting is evaluated, things should work as documented:

Command-line         Using superproject  → `current_branch_only`
------------         ------------------  -----------------------
                     No                  From manifest
                     Yes                 True
--current-branch     No                  True
--current-branch     Yes                 True
--no-current-branch  No                  False
--no-current-branch  Yes                 True

Change-Id: I933c232d2fbecc6b9bdc364ebac181798bce9175
Tested-by: Daniel Andersson <daniel.r.andersson@volvocars.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334270
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -280,8 +280,13 @@ later is required to fix a server side protocol bug.
     return branch
 
   def _GetCurrentBranchOnly(self, opt):
-    """Returns True if current-branch or use-superproject options are enabled."""
-    return opt.current_branch_only or git_superproject.UseSuperproject(opt, self.manifest)
+    """Returns whether current-branch or use-superproject options are enabled.
+
+    Returns:
+      True if a superproject is requested, otherwise the value of the
+      current_branch option (True, False or None).
+    """
+    return git_superproject.UseSuperproject(opt, self.manifest) or opt.current_branch_only
 
   def _UpdateProjectsRevisionId(self, opt, args, load_local_manifests, superproject_logging_data):
     """Update revisionId of every project with the SHA from superproject.