[PATCH] log --format: teach %C(auto,black) to paint it black only on terminals

[Junio C Hamano <gitster@pobox.com>]

Traditionally, %C(color attr) always emitted the ANSI color
sequence; it was up to the scripts that wanted to conditionally
color their output to omit %C(...) specifier when they do not want
colors.

Optionally allow "auto," to be prefixed to the color, so that the
output is colored iff it goes to the terminal.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 * This time with minimum test and documentation.

 Documentation/pretty-formats.txt |  4 +++-
 pretty.c                         | 13 ++++++++++---
 t/t6006-rev-list-format.sh       | 30 ++++++++++++++++++++++++++----
 3 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d9edded..2af017c 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -144,7 +144,9 @@ The placeholders are:
 - '%Cgreen': switch color to green
 - '%Cblue': switch color to blue
 - '%Creset': reset color
-- '%C(...)': color specification, as described in color.branch.* config option
+- '%C(...)': color specification, as described in color.branch.* config option;
+  adding `auto,` at the beginning will emit color only when the
+  output is going to a terminal
 - '%m': left, right or boundary mark
 - '%n': newline
 - '%%': a raw '%'
diff --git a/pretty.c b/pretty.c
index dba6828..b9fd972 100644
--- a/pretty.c
+++ b/pretty.c
@@ -960,12 +960,19 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 	switch (placeholder[0]) {
 	case 'C':
 		if (placeholder[1] == '(') {
-			const char *end = strchr(placeholder + 2, ')');
+			const char *begin = placeholder + 2;
+			const char *end = strchr(begin, ')');
 			char color[COLOR_MAXLEN];
+
 			if (!end)
 				return 0;
-			color_parse_mem(placeholder + 2,
-					end - (placeholder + 2),
+			if (!memcmp(begin, "auto,", 5)) {
+				if (!want_color(-1))
+					return end - placeholder + 1;
+				begin += 5;
+			}
+			color_parse_mem(begin,
+					end - begin,
 					"--pretty format", color);
 			strbuf_addstr(sb, color);
 			return end - placeholder + 1;
diff --git a/t/t6006-rev-list-format.sh b/t/t6006-rev-list-format.sh
index f94f0c4..bfcc1c6 100755
--- a/t/t6006-rev-list-format.sh
+++ b/t/t6006-rev-list-format.sh
@@ -11,12 +11,12 @@ touch foo && git add foo && git commit -m "added foo" &&
 '
 
 # usage: test_format name format_string <expected_output
-test_format() {
+test_format () {
 	cat >expect.$1
 	test_expect_success "format $1" "
-git rev-list --pretty=format:'$2' master >output.$1 &&
-test_cmp expect.$1 output.$1
-"
+		git rev-list --pretty=format:'$2'${3:+ $3} master >output.$1 &&
+		test_cmp expect.$1 output.$1
+	"
 }
 
 test_format percent %%h <<'EOF'
@@ -124,6 +124,28 @@ commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
 ^[[1;31;43mfoo^[[m
 EOF
 
+test_format advanced-colors-auto \
+	'%C(auto,red yellow bold)foo%C(auto,reset)' --color <<'EOF'
+commit 131a310eb913d107dd3c09a65d1651175898735d
+foo
+commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
+foo
+EOF
+
+# %C(auto,...) should trump --color=always
+#
+# NEEDSWORK: --color=never should also be tested but we need to run a
+# similar test under pseudo-terminal with test_terminal which is too
+# much hassle for its worth.
+
+test_format advanced-colors-forced \
+	'%C(auto,red yellow bold)foo%C(auto,reset)' --color=always <<'EOF'
+commit 131a310eb913d107dd3c09a65d1651175898735d
+foo
+commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
+foo
+EOF
+
 cat >commit-msg <<'EOF'
 Test printing of complex bodies
 
-- 
1.8.1.rc2.126.ge9b7782