Store up to 30000 packet log lines for git fetch.

We currently fetch tags, which consumes ~17k lines. This limit ensures that we
won't blow up the log files in exceptional circumstances [currently setting
around ~2MB], but still allows us to get all the logs.

Change-Id: Ib690aaa07e2bde8549d221b90511b6c4863c3358
Recipe-Nontrivial-Roll: chromiumos
Recipe-Nontrivial-Roll: skia
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1867971
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>

git_cache.py

@@ -558,16 +558,16 @@ class Mirror(object):
             'GIT_TRACE_PERFORMANCE': '1',
             'GIT_TRACE_SETUP': '1'
         })
-        # Only print first 30 packets. We can use nonlocal keyword once we
+        # Only print first 30000 packets. We can use nonlocal keyword once we
         # switch to python 3.
         packet_count = [0]
 
         def FilterPacket(log_line):
           if 'packet:' in log_line:
             packet_count[0] += 1
-            if packet_count[0] == 30:
+            if packet_count[0] == 30000:
               self.print('Truncating remaining packets')
-            if packet_count[0] >= 30:
+            if packet_count[0] >= 30000:
               return
           self.print(log_line)