Create a new manpage for the gitignore format, and reference it elsewhere

Create a new manpage for the gitignore format, and reference it elsewhere

Only git-ls-files(1) describes the gitignore format in detail, and it does so
with reference to git-ls-files options.  Most users don't use the plumbing
command git-ls-files directly, and shouldn't have to look in its manpage for
information on the gitignore format.

Create a new manpage gitignore(5) (Documentation/gitignore.txt), and factor
out the gitignore documentation into that file, changing it to refer to
.gitignore and $GIT_DIR/info/exclude as used by porcelain commands.  Reference
gitignore(5) from other relevant manpages and documentation.  Remove
now-redundant information on exclude patterns from git-ls-files(1), leaving
only information on how git-ls-files options specify exclude patterns and what
precedence they have.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>