diff: add support for --color

* doc/diffutils.texi (diff Options): Add documentation for --color.
Copied from coreutils ls --color.
* src/context.c (pr_unidiff_hunk): Set the color context.
(print_context_header): Likewise.
(pr_context_hunk): Likewise.
* src/diff.h (enum colors_style): New enum to record when to use colors.
(colors_style): New variable to memorize the argument value.
(set_color_context): Add function definition.
* src/diff.c: : Define COLOR_OPTION.
(specify_colors_style): New function.
(longopts): Add --color.
(main): Handle --color argument.
(option_help_msgid): Add usage string for --color.
* src/normal.c (print_normal_hunk): Set the color context.
* src/side.c (print_1sdiff_line): Likewise.
* src/util.c (print_1_line_nl): New function.
(print_1_line): Make it a wrapper of 'print_1_line_nl'.
(colors_enabled): New boolean variable.
(begin_output): Call check_color_output once the output file is
configured.
(output_1_line): Periodically call `process_signals'.
(caught_signals): New sigset_t.
(colors_enabled): New boolean variable.
(interrupt_signal): New sig_atomic_t.
(stop_signal_count): New sig_atomic_t.
(check_color_output): New function.
(install_signal_handlers): Likewise. Copied from coreutils ls.
(process_signals): Likewise.  Copied from coreutils ls.
(set_color_context): New function.
(sighandler): Likewise.  Copied from coreutils ls.
(stophandler): Likewise.  Copied from coreutils ls.

1 files changed, 21 insertions, 0 deletions

diff --git a/doc/diffutils.texi b/doc/diffutils.texi
index 091257f..b2c39da 100644
--- a/doc/diffutils.texi
+++ b/doc/diffutils.texi
@@ -3742,6 +3742,27 @@ Read and write data in binary mode.  @xref{Binary}.
 Use the context output format, showing three lines of context.
 @xref{Context Format}.
 
+@item --color [=@var{when}]
+@cindex color, distinguishing different context
+Specify whether to use color for distinguishing different contexts,
+like header, added or removed lines.  @var{when} may be omitted, or
+one of:
+@itemize @bullet
+@item none
+@vindex none @r{color option}
+Do not use color at all.  This is the default when no --color option
+is specified.
+@item auto
+@vindex auto @r{color option}
+@cindex terminal, using color iff
+Use color only if standard output is a terminal.
+@item always
+@vindex always @r{color option}
+Always use color.
+@end itemize
+Specifying @option{--color} and no @var{when} is equivalent to
+@option{--color=auto}.
+
 @item -C @var{lines}
 @itemx --context@r{[}=@var{lines}@r{]}
 Use the context output format, showing @var{lines} (an integer) lines of