PushCommand: allow users to disable use of bitmaps for push

PushCommand: allow users to disable use of bitmaps for push

Reachability bitmaps are designed to speed up the "counting objects"
phase of generating a pack during a clone or fetch. They are not
optimized for Git clients sending a small topic branch via "git push".
In some cases (see [1]), using reachability bitmaps during "git push"
can cause significant performance regressions.

Add PushCommand#setUseBitmaps(boolean) to allow users to tell "git push"
not to use bitmaps.

[1]: https://lore.kernel.org/git/87zhoz8b9o.fsf@evledraar.gmail.com/

Change-Id: I7fb7d26084ec63ddfa7249cf58abb85929b30e56
Signed-off-by: kylezhao <kylezhao@tencent.com>