Set a message when we fail to lock.

Also make the put function void, since it's called from free, which
cannot report errors. The only errors we can experience here are
internal state corruption, so we assert that we are trying to put a
pack which we have previously got.

As reported by coverity, we would leak some memory in error conditions.

If we fail to insert the packfile in the map, make sure to free it.

This makes the free function only attempt to remove its mwindows from
the global list if we have opened the packfile to avoid accessing the
list unlocked.

Git for Windows 1.9.4 changed the behavior when the text=auto
attribute is specified and core.autocrlf=false.  Previous observed
behavior would *not* filter files when going into the working
directory, the new behavior *does* filter.  Update our behavior to match.

When checking out files, we're performing conversion into the user's
native line endings, but we only want to do it for files which have
consistent line endings. Refuse to perform the conversion for mixed-EOL
files.

The CRLF->LF filter is left as-is, as that conversion is considered to be
normalization by git and should force a conversion of the line endings.

Opening the same repository multiple times will currently open the same
file multiple times, as well as map the same region of the file multiple
times. This is not necessary, as the packfile data is immutable.

Instead of opening and closing packfiles directly, introduce an
indirection and allocate packfiles globally. This does mean locking on
each packfile open, but we already use this lock for the global mwindow
list so it doesn't introduce a new contention point.