abbrev cleanup: use symbolic constants

The minimum length of abbreviated object name was hardcoded in
different places to be 4, risking inconsistencies in the future.
Also there were three different "default abbreviation
precision".  Use two C preprocessor symbols to clean up this
mess.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -221,6 +221,9 @@ extern int has_pack_file(const unsigned char *sha1);
 extern int has_pack_index(const unsigned char *sha1);
 
 /* Convert to/from hex/sha1 representation */
+#define MINIMUM_ABBREV 4
+#define DEFAULT_ABBREV 7
+
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern int get_sha1_hex(const char *hex, unsigned char *sha1);
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */

describe.c

@@ -11,7 +11,6 @@ static const char describe_usage[] =
 static int all = 0;	/* Default to annotated tags only */
 static int tags = 0;	/* But allow any tags if --tags is specified */
 
-#define DEFAULT_ABBREV 8 /* maybe too many */
 static int abbrev = DEFAULT_ABBREV;
 
 static int names = 0, allocs = 0;
@@ -155,7 +154,7 @@ int main(int argc, char **argv)
 			tags = 1;
 		else if (!strncmp(arg, "--abbrev=", 9)) {
 			abbrev = strtoul(arg + 9, NULL, 10);
-			if (abbrev < 4 || 40 <= abbrev)
+			if (abbrev < MINIMUM_ABBREV || 40 <= abbrev)
 				abbrev = DEFAULT_ABBREV;
 		}
 		else

diff.c

@@ -723,7 +723,7 @@ static void run_diff(struct diff_filepair *p, struct diff_options *o)
 
 	if (memcmp(one->sha1, two->sha1, 20)) {
 		char one_sha1[41];
-		int abbrev = o->full_index ? 40 : DIFF_DEFAULT_INDEX_ABBREV;
+		int abbrev = o->full_index ? 40 : DEFAULT_ABBREV;
 		memcpy(one_sha1, sha1_to_hex(one->sha1), 41);
 
 		len += snprintf(msg + len, sizeof(msg) - len,
@@ -846,7 +846,7 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 	else if (!strcmp(arg, "--find-copies-harder"))
 		options->find_copies_harder = 1;
 	else if (!strcmp(arg, "--abbrev"))
-		options->abbrev = DIFF_DEFAULT_ABBREV;
+		options->abbrev = DEFAULT_ABBREV;
 	else if (!strncmp(arg, "--abbrev=", 9))
 		options->abbrev = strtoul(arg + 9, NULL, 10);
 	else

diff.h

@@ -88,9 +88,6 @@ extern int diff_setup_done(struct diff_options *);
 
 #define DIFF_PICKAXE_ALL	1
 
-#define DIFF_DEFAULT_INDEX_ABBREV	7 /* hex digits */
-#define DIFF_DEFAULT_ABBREV	7 /* hex digits */
-
 extern void diffcore_std(struct diff_options *);
 
 extern void diffcore_std_no_resolve(struct diff_options *);

sha1_name.c

@@ -155,7 +155,7 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
 	char canonical[40];
 	unsigned char res[20];
 
-	if (len < 4)
+	if (len < MINIMUM_ABBREV)
 		return -1;
 	memset(res, 0, 20);
 	memset(canonical, 'x', 40);