make lineno_width() from blame reusable for others

builtin/blame.c has a helper function to compute how many columns
we need to show a line-number, whose implementation is reusable as
a more generic helper function to count the number of columns
necessary to show any cardinal number.

Rename it to decimal_width(), move it to pager.c and export it for
use by future callers.
Signed-off-by: NZbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -1828,18 +1828,6 @@ static int read_ancestry(const char *graft_file)
 	return 0;
 }
 
-/*
- * How many columns do we need to show line numbers in decimal?
- */
-static int lineno_width(int lines)
-{
-	int i, width;
-
-	for (width = 1, i = 10; i <= lines; width++)
-		i *= 10;
-	return width;
-}
-
 /*
  * How many columns do we need to show line numbers, authors,
  * and filenames?
@@ -1880,9 +1868,9 @@ static void find_alignment(struct scoreboard *sb, int *option)
 		if (largest_score < ent_score(sb, e))
 			largest_score = ent_score(sb, e);
 	}
-	max_orig_digits = lineno_width(longest_src_lines);
-	max_digits = lineno_width(longest_dst_lines);
-	max_score_digits = lineno_width(largest_score);
+	max_orig_digits = decimal_width(longest_src_lines);
+	max_digits = decimal_width(longest_dst_lines);
+	max_score_digits = decimal_width(largest_score);
 }
 
 /*

cache.h

@@ -1176,6 +1176,7 @@ extern void setup_pager(void);
 extern const char *pager_program;
 extern int pager_in_use(void);
 extern int pager_use_color;
+extern int decimal_width(int);
 
 extern const char *editor_program;
 extern const char *askpass_program;

pager.c

@@ -110,3 +110,15 @@ int pager_in_use(void)
 	env = getenv("GIT_PAGER_IN_USE");
 	return env ? git_config_bool("GIT_PAGER_IN_USE", env) : 0;
 }
+
+/*
+ * How many columns do we need to show this number in decimal?
+ */
+int decimal_width(int number)
+{
+	int i, width;
+
+	for (width = 1, i = 10; i <= number; width++)
+		i *= 10;
+	return width;
+}