doc: document backslash in gitignore patterns

Because gitignore patterns are passed to fnmatch, the handling of
backslashes is the same as it is there: it can be used to escape
metacharacters. We do reference fnmatch(3) for more details, but it may
be friendlier to point out this implication explicitly (especially for
people who want to know about backslash handling and search the
documentation for that word). There are also two cases that I've seen
some other backslash-escaping systems handle differently, so let's
describe those:

  1. A backslash before any character treats that character literally,
     even if it's not otherwise a meta-character. As opposed to
     including the backslash itself (like "foo\bar" in shell expands to
     "foo\bar") or forbidding it ("foo\zar" is required to produce a
     diagnostic in C).

  2. A backslash at the end of the string is an invalid pattern (and not
     a literal backslash).

This second one in particular was a point of confusion between our
implementation and the one in JGit. Our wildmatch behavior matches what
POSIX specifies for fnmatch, so the code and documentation are in line.
But let's add a test to cover this case. Note that the behavior here
differs between wildmatch itself (which is what gitignore will use) and
pathspec matching (which will only turn to wildmatch if a literal match
fails). So we match "foo\" to "foo\" in pathspecs, but not via
gitignore.

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

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