interpret_branch_name: always respect "namelen" parameter

interpret_branch_name gets passed a "name" buffer to parse,
along with a "namelen" parameter representing its length. If
"namelen" is zero, we fallback to the NUL-terminated
string-length of "name".

However, it does not necessarily follow that if we have
gotten a non-zero "namelen", it is the NUL-terminated
string-length of "name". E.g., when get_sha1() is parsing
"foo:bar", we will be asked to operate only on the first
three characters.

Yet in interpret_branch_name and its helpers, we use string
functions like strchr() to operate on "name", looking past
the length we were given.  This can result in us mis-parsing
object names.  We should instead be limiting our search to
"namelen" bytes.

There are three distinct types of object names this patch
addresses:

  - The intrepret_empty_at helper uses strchr to find the
    next @-expression after our potential empty-at.  In an
    expression like "@:foo@bar", it erroneously thinks that
    the second "@" is relevant, even if we were asked only
    to look at the first character. This case is easy to
    trigger (and we test it in this patch).

  - When finding the initial @-mark for @{upstream}, we use
    strchr.  This means we might treat "foo:@{upstream}" as
    the upstream for "foo:", even though we were asked only
    to look at "foo". We cannot test this one in practice,
    because it is masked by another bug (which is fixed in
    the next patch).

  - The interpret_nth_prior_checkout helper did not receive
    the name length at all. This turns out not to be a
    problem in practice, though, because its parsing is so
    limited: it always starts from the far-left of the
    string, and will not tolerate a colon (which is
    currently the only way to get a smaller-than-strlen
    "namelen"). However, it's still worth fixing to make the
    code more obviously correct, and to future-proof us
    against callers with more exotic buffers.

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

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