add git-check-ignore sub-command

This works in a similar manner to git-check-attr.

Thanks to Jeff King and Junio C Hamano for the idea:
http://thread.gmane.org/gmane.comp.version-control.git/108671/focus=108815

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 89 insertions, 0 deletions

diff --git a/Documentation/git-check-ignore.txt b/Documentation/git-check-ignore.txt
new file mode 100644
index 0000000000..854e4d0c42
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+git-check-ignore(1)
+===================
+
+NAME
+----
+git-check-ignore - Debug gitignore / exclude files
+
+
+SYNOPSIS
+--------
+[verse]
+'git check-ignore' [options] pathname...
+'git check-ignore' [options] --stdin < <list-of-paths>
+
+DESCRIPTION
+-----------
+
+For each pathname given via the command-line or from a file via
+`--stdin`, show the pattern from .gitignore (or other input files to
+the exclude mechanism) that decides if the pathname is excluded or
+included.  Later patterns within a file take precedence over earlier
+ones.
+
+OPTIONS
+-------
+-q, --quiet::
+	Don't output anything, just set exit status.  This is only
+	valid with a single pathname.
+
+-v, --verbose::
+	Also output details about the matching pattern (if any)
+	for each given pathname.
+
+--stdin::
+	Read file names from stdin instead of from the command-line.
+
+-z::
+	The output format is modified to be machine-parseable (see
+	below).  If `--stdin` is also given, input paths are separated
+	with a NUL character instead of a linefeed character.
+
+OUTPUT
+------
+
+By default, any of the given pathnames which match an ignore pattern
+will be output, one per line.  If no pattern matches a given path,
+nothing will be output for that path; this means that path will not be
+ignored.
+
+If `--verbose` is specified, the output is a series of lines of the form:
+
+<source> <COLON> <linenum> <COLON> <pattern> <HT> <pathname>
+
+<pathname> is the path of a file being queried, <pattern> is the
+matching pattern, <source> is the pattern's source file, and <linenum>
+is the line number of the pattern within that source.  If the pattern
+contained a `!` prefix or `/` suffix, it will be preserved in the
+output.  <source> will be an absolute path when referring to the file
+configured by `core.excludesfile`, or relative to the repository root
+when referring to `.git/info/exclude` or a per-directory exclude file.
+
+If `-z` is specified, the pathnames in the output are delimited by the
+null character; if `--verbose` is also specified then null characters
+are also used instead of colons and hard tabs:
+
+<source> <NULL> <linenum> <NULL> <pattern> <NULL> <pathname> <NULL>
+
+
+EXIT STATUS
+-----------
+
+0::
+	One or more of the provided paths is ignored.
+
+1::
+	None of the provided paths are ignored.
+
+128::
+	A fatal error was encountered.
+
+SEE ALSO
+--------
+linkgit:gitignore[5]
+linkgit:gitconfig[5]
+linkgit:git-ls-files[5]
+
+GIT
+---
+Part of the linkgit:git[1] suite