diff: diff.context configuration gives default to -U

Introduce a configuration variable diff.context that tells
Porcelain commands to use a non-default number of context
lines instead of 3 (the default).  With this variable, users
do not have to keep repeating "git log -U8" from the command
line; instead, it becomes sufficient to say "git config
diff.context 8" just once.

Signed-off-by: Jeff Muizelaar <jmuizelaar@mozilla.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -56,6 +56,10 @@ diff.statGraphWidth::
 	Limit the width of the graph part in --stat output. If set, applies
 	to all commands generating --stat output except format-patch.
 
+diff.context::
+	Generate diffs with <n> lines of context instead of the default of
+	3. This value is overridden by the -U option.
+
 diff.external::
 	If this config variable is set, diff generation is not
 	performed using the internal diff machinery, but using the

diff.c

@@ -26,6 +26,7 @@ static int diff_detect_rename_default;
 static int diff_rename_limit_default = 400;
 static int diff_suppress_blank_empty;
 static int diff_use_color_default = -1;
+static int diff_context_default = 3;
 static const char *diff_word_regex_cfg;
 static const char *external_diff_cmd_cfg;
 int diff_auto_refresh_index = 1;
@@ -141,6 +142,12 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 		diff_use_color_default = git_config_colorbool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "diff.context")) {
+		diff_context_default = git_config_int(var, value);
+		if (diff_context_default < 0)
+			return -1;
+		return 0;
+	}
 	if (!strcmp(var, "diff.renames")) {
 		diff_detect_rename_default = git_config_rename(var, value);
 		return 0;
@@ -3170,7 +3177,7 @@ void diff_setup(struct diff_options *options)
 	options->break_opt = -1;
 	options->rename_limit = -1;
 	options->dirstat_permille = diff_dirstat_permille_default;
-	options->context = 3;
+	options->context = diff_context_default;
 	DIFF_OPT_SET(options, RENAME_EMPTY);
 
 	options->change = diff_change;

t/t4055-diff-context.sh

@@ -0,0 +1,93 @@
+#!/bin/sh
+#
+# Copyright (c) 2012 Mozilla Foundation
+#
+
+test_description='diff.context configuration'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	cat >x <<-\EOF &&
+	firstline
+	b
+	c
+	d
+	e
+	f
+	preline
+	postline
+	i
+	j
+	k
+	l
+	m
+	n
+	EOF
+	git update-index --add x &&
+	git commit -m initial &&
+
+	git cat-file blob HEAD:x |
+	sed "/preline/a\
+	ADDED" >x &&
+	git update-index --add x &&
+	git commit -m next &&
+
+	git cat-file blob HEAD:x |
+	sed s/ADDED/MODIFIED/ >x
+'
+
+test_expect_success 'the default number of context lines is 3' '
+	git diff >output &&
+	! grep "^ d" output &&
+	grep "^ e" output &&
+	grep "^ j" output &&
+	! grep "^ k" output
+'
+
+test_expect_success 'diff.context honored by "log"' '
+	git log -1 -p >output &&
+	! grep firstline output &&
+	git config diff.context 8 &&
+	git log -1 -p >output &&
+	grep "^ firstline" output
+'
+
+test_expect_success 'The -U option overrides diff.context' '
+	git config diff.context 8 &&
+	git log -U4 -1 >output &&
+	! grep "^ firstline" output
+'
+
+test_expect_success 'diff.context honored by "diff"' '
+	git config diff.context 8 &&
+	git diff >output &&
+	grep "^ firstline" output
+'
+
+test_expect_success 'plumbing not affected' '
+	git config diff.context 8 &&
+	git diff-files -p >output &&
+	! grep "^ firstline" output
+'
+
+test_expect_success 'non-integer config parsing' '
+	git config diff.context no &&
+	test_must_fail git diff 2>output &&
+	test_i18ngrep "bad config value" output
+'
+
+test_expect_success 'negative integer config parsing' '
+	git config diff.context -1 &&
+	test_must_fail git diff 2>output &&
+	test_i18ngrep "bad config file" output
+'
+
+test_expect_success '-U0 is valid, so is diff.context=0' '
+	git config diff.context 0 &&
+	git diff >output &&
+	grep "^-ADDED" output &&
+	grep "^+MODIFIED" output
+'
+
+test_done