color: support "italic" attribute

We already support bold, underline, and similar attributes.
Let's add italic to the mix.  According to the Wikipedia
page on ANSI colors, this attribute is "not widely
supported", but it does seem to work on my xterm.

We don't have to bump the maximum color size because we were
already over-allocating it (but we do adjust the comment
appropriately).

Requested-by: Simon Courtois <scourtois@cubyx.fr>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -160,10 +160,11 @@ Colors may also be given as numbers between 0 and 255; these use ANSI
 your terminal supports it, you may also specify 24-bit RGB values as
 hex, like `#ff0ab3`.
 +
-The accepted attributes are `bold`, `dim`, `ul`, `blink`, and `reverse`.
-The position of any attributes with respect to the colors (before, after,
-or in between), doesn't matter. Specific attributes may be turned off
-by prefixing them with `no` or `no-` (e.g., `noreverse`, `no-ul`, etc).
+The accepted attributes are `bold`, `dim`, `ul`, `blink`, `reverse`, and
+`italic`.  The position of any attributes with respect to the colors
+(before, after, or in between), doesn't matter. Specific attributes may
+be turned off by prefixing them with `no` or `no-` (e.g., `noreverse`,
+`no-ul`, etc).
 +
 For git's pre-defined color slots, the attributes are meant to be reset
 at the beginning of each item in the colored output. So setting

color.c

@@ -133,6 +133,7 @@ static int parse_attr(const char *name, size_t len)
 #define ATTR(x, val, neg) { (x), sizeof(x)-1, (val), (neg) }
 		ATTR("bold",      1, 22),
 		ATTR("dim",       2, 22),
+		ATTR("italic",    3, 23),
 		ATTR("ul",        4, 24),
 		ATTR("blink",     5, 25),
 		ATTR("reverse",   7, 27)

color.h

@@ -16,7 +16,8 @@ struct strbuf;
  *
  * The space for attributes is also slightly overallocated, as
  * the negation for some attributes is the same (e.g., nobold and nodim).
- * We also allocate space for 6 attributes (even though we have only 5).
+ *
+ * We allocate space for 6 attributes.
  */
 #define COLOR_MAXLEN 70
 

t/t4026-color.sh

@@ -60,8 +60,9 @@ test_expect_success 'long color specification' '
 
 test_expect_success 'absurdly long color specification' '
 	color \
-	  "#ffffff #ffffff bold nobold dim nodim ul noul blink noblink reverse noreverse" \
-	  "[1;2;4;5;7;22;24;25;27;38;2;255;255;255;48;2;255;255;255m"
+	  "#ffffff #ffffff bold nobold dim nodim italic noitalic
+	   ul noul blink noblink reverse noreverse" \
+	  "[1;2;3;4;5;7;22;23;24;25;27;38;2;255;255;255;48;2;255;255;255m"
 '
 
 test_expect_success '0-7 are aliases for basic ANSI color names' '