Reduce memory usage of fast-import.

Some structs are allocated rather frequently, but were using integer
types which were far larger than required to actually store their
full value range.

As packfiles are limited to 4 GiB we don't need more than 32 bits to
store the offset of an object within that packfile, an `unsigned long`
on a 64 bit system is likely a 64 bit unsigned value.  Saving 4 bytes
per object on a 64 bit system can add up fast on any sizable import.

As atom strings are strictly single components in a path name these
are probably limited to just 255 bytes by the underlying OS.  Going
to that short of a string is probably too restrictive, but certainly
`unsigned int` is far too large for their lengths.  `unsigned short`
is a reasonable limit.

Modes within a tree really only need two bytes to store their whole
value; using `unsigned int` here is vast overkill.  Saving 4 bytes
per file entry in an active branch can add up quickly on a project
with a large number of files.

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

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