git_command: fix input passing

After reworking this function to use subprocess for output capturing
in commit c87c1863b1 ("git_command:
switch process capturing over to subprocess"), passing input via
stdin write no longer works.  We have to pass it via communicate(),
and we have to pass it a string instead of bytes (since we always
use encoding='utf-8' now).  This is fine since the only user of the
input= setting today is already passing in a string.

Bug: https://crbug.com/gerrit/16151
Change-Id: Ic58db1e568b8f8aa840a6d62c5a157c14aa6d9bc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/343515
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>

git_command.py

@@ -281,14 +281,9 @@ class GitCommand(object):
       ssh_proxy.add_client(p)
 
     self.process = p
-    if input:
-      if isinstance(input, str):
-        input = input.encode('utf-8')
-      p.stdin.write(input)
-      p.stdin.close()
 
     try:
-      self.stdout, self.stderr = p.communicate()
+      self.stdout, self.stderr = p.communicate(input=input)
     finally:
       if ssh_proxy:
         ssh_proxy.remove_client(p)