t: enable ASan's strict_string_checks option

t: enable ASan's strict_string_checks option

ASan has an option to enable strict string checking, where any pointer
passed to a function that expects a NUL-terminated string will be
checked for that NUL termination. This can sometimes produce false
positives. E.g., it is not wrong to pass a buffer with { '1', '2', '\n' }
into strtoul(). Even though it is not NUL-terminated, it will stop at
the newline.

But in trying it out, it identified two problematic spots in our test
suite (which have now been adjusted):

  1. The strtol() parsing in cache-tree.c was a real potential problem,
     which would have been very hard to find otherwise (since it
     required constructing a very specific broken index file).

  2. The use of string functions in fsck_ident() were false positives,
     because we knew that there was always a trailing newline which
     would stop the functions from reading off the end of the buffer.
     But the reasoning behind that is somewhat fragile, and silencing
     those complaints made the code easier to reason about.

So even though this did not find any earth-shattering bugs, and even had
a few false positives, I'm sufficiently convinced that its complaints
are more helpful than hurtful. Let's turn it on by default (since the
test suite now runs cleanly with it) and see if it ever turns up any
other instances.

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