Use more restrictive patterns for sideband progress scraping

To avoid scraping a non-progress message as though it were a progress
item for the progress monitor, use a more restrictive pattern to
watch the remote side's messages.  These two regexps should match
any message produced by C Git since 42e18fbf5f94 ("more compact
progress display", Oct 2007), and which first appeared in Git 1.5.4.

Change-Id: I57e34cf59d42c1dbcbd1a83dd6f499ce5e39d15d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>