start: do not swallow git output all the time

Normally git produces no output when creating or switching branches.
If there's a problem though, we want to show that to the user.  So
switch from capturing all output to running in quiet mode.

Bug: https://crbug.com/gerrit/15819
Change-Id: I7873ecc7c3bacce591899cc9471cb0244eb74541
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343454
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/project.py b/project.py
index 4bf1eca..894fb38 100644
--- a/project.py
+++ b/project.py
@@ -1678,10 +1678,7 @@
 
     all_refs = self.bare_ref.all
     if R_HEADS + name in all_refs:
-      return GitCommand(self,
-                        ['checkout', name, '--'],
-                        capture_stdout=True,
-                        capture_stderr=True).Wait() == 0
+      return GitCommand(self, ['checkout', '-q', name, '--']).Wait() == 0
 
     branch = self.GetBranch(name)
     branch.remote = self.GetRemote()
@@ -1706,10 +1703,7 @@
       branch.Save()
       return True
 
-    if GitCommand(self,
-                  ['checkout', '-b', branch.name, revid],
-                  capture_stdout=True,
-                  capture_stderr=True).Wait() == 0:
+    if GitCommand(self, ['checkout', '-q', '-b', branch.name, revid]).Wait() == 0:
       branch.Save()
       return True
     return False