filter-repo: fix possible deadlock in sanity_check_args

I'm a little surprised that stdout buffers must have filled up on MacOS X, but
either way we don't have to wait for the '-h' processes to finish before
attempting to read stdout.  In fact, since we weren't storing the returncode
attribute from calling p.wait(), there wasn't much point in doing so.  Trying
to read all stdout all at once is going to implicitly take until the process
finishes anyway, so just do that.

Reported-by: Benoit Lefèvre <contact@benoit-lefevre.org>
Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -2039,7 +2039,6 @@ EXAMPLES
     # PERF: remove these checks once new enough git versions are common
     p = subproc.Popen('git fast-export -h'.split(),
                       stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-    p.wait()
     output = p.stdout.read()
     if b'--mark-tags' not in output: # pragma: no cover
       global write_marks
@@ -2060,7 +2059,6 @@ EXAMPLES
       # diff-tree --combined-all-paths, which is even more important...
       p = subproc.Popen('git diff-tree -h'.split(),
                         stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
-      p.wait()
       output = p.stdout.read()
       if b'--combined-all-paths' not in output:
         # We need a version of git-diff-tree with --combined-all-paths