match_pathname(): give fnmatch one char of prefix context

In match_pathname(), which we use for matching .gitignore and
.gitattribute patterns, we are comparing paths with fnmatch patterns
(actually our extended wildmatch, which will be important).  There's an
extra optimization there: we pre-compute the number of non-wildcard
characters at the beginning of the pattern and do an fspathncmp() on
that prefix.

That lets us avoid fnmatch entirely on patterns without wildcards, and
shrinks the amount of work we hand off to fnmatch. For a pattern like
"foo*.txt" and a path "foobar.txt", we'd cut away the matching "foo"
prefix and just pass "*.txt" and "bar.txt" to fnmatch().

But this misses a subtle corner case. In fnmatch(), we'll think
"bar.txt" is the start of the path, but it's not. This doesn't matter
for the pattern above, but consider the wildmatch pattern "foo**/bar"
and the path "foobar". These two should not match, because there is no
file named "bar", and the "**" applies only to the containing directory
name. But after removing the "foo" prefix, fnmatch will get "**/bar" and
"bar", which it does consider a match, because "**/" can match zero
directories.

We can solve this by giving fnmatch a bit more context. As long as it
has one byte of the matched prefix, then it will know that "bar" is not
the start of the path. In this example it would get "o**/bar" and
"obar", and realize that they cannot match.

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

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