fast-export: tighten anonymize_mem() interface to handle only strings

fast-export: tighten anonymize_mem() interface to handle only strings

While the anonymize_mem() interface _can_ store arbitrary byte
sequences, none of the callers uses this feature (as of the previous
commit). We'd like to keep it that way, as we'll be exposing the
string-like nature of the anonymization routines to the user. So let's
tighten up the interface a bit:

  - don't treat "len" as an out-parameter from anonymize_mem(); this
    ensures callers treat the pointer result as a NUL-terminated string

  - likewise, don't treat "len" as an out-parameter from generator
    functions

  - swap out "void *" for "char *" as appropriate to signal that we
    don't handle arbitrary memory

  - rename the function to anonymize_str()

This will also open up some optimization opportunities in a future
patch.

Note that we can't drop the "len" parameter entirely. Some callers do
pass in partial strings (e.g., "foo/bar", len=3) to avoid copying, and
we need to handle those still.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>