abbrev: add FALLBACK_DEFAULT_ABBREV to prepare for auto sizing

We'll be introducing a new way to decide the default abbreviation
length by initialising DEFAULT_ABBREV to -1 to signal the first call
to "find unique abbreviation" codepath to compute a reasonable value
based on the number of objects we have to avoid collisions.

We have long relied on DEFAULT_ABBREV being a positive concrete
value that is used as the abbreviation length when no extra
configuration or command line option has overridden it.  Some
codepaths wants to use such a positive concrete default value
even before making their first request to actually trigger the
computation for the auto sized default.

Introduce FALLBACK_DEFAULT_ABBREV and use it to the code that
attempts to align the report from "git fetch".  For now, this
macro is also used to initialize the default_abbrev variable,
but the auto-sizing code will use -1 and then use the value of
FALLBACK_DEFAULT_ABBREV as the starting point of auto-sizing.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -17,6 +17,9 @@
 #include "argv-array.h"
 #include "utf8.h"
 
+#define TRANSPORT_SUMMARY(x) \
+	(int)(TRANSPORT_SUMMARY_WIDTH + strlen(x) - gettext_width(x)), (x)
+
 static const char * const builtin_fetch_usage[] = {
 	N_("git fetch [<options>] [<repository> [<refspec>...]]"),
 	N_("git fetch [<options>] <group>"),

cache.h

@@ -1183,6 +1183,9 @@ static inline int hex2chr(const char *s)
 #define MINIMUM_ABBREV minimum_abbrev
 #define DEFAULT_ABBREV default_abbrev
 
+/* used when the code does not know or care what the default abbrev is */
+#define FALLBACK_DEFAULT_ABBREV 7
+
 struct object_context {
 	unsigned char tree[20];
 	char path[PATH_MAX];

environment.c

@@ -16,7 +16,7 @@ int trust_executable_bit = 1;
 int trust_ctime = 1;
 int check_stat = 1;
 int has_symlinks = 1;
-int minimum_abbrev = 4, default_abbrev = 7;
+int minimum_abbrev = 4, default_abbrev = FALLBACK_DEFAULT_ABBREV;
 int ignore_case;
 int assume_unchanged;
 int prefer_symlink_refs;

transport.h

@@ -142,8 +142,7 @@ struct transport {
 #define TRANSPORT_PUSH_ATOMIC 8192
 #define TRANSPORT_PUSH_OPTIONS 16384
 
-#define TRANSPORT_SUMMARY_WIDTH (2 * DEFAULT_ABBREV + 3)
-#define TRANSPORT_SUMMARY(x) (int)(TRANSPORT_SUMMARY_WIDTH + strlen(x) - gettext_width(x)), (x)
+#define TRANSPORT_SUMMARY_WIDTH (2 * FALLBACK_DEFAULT_ABBREV + 3)
 
 /* Returns a transport suitable for the url */
 struct transport *transport_get(struct remote *, const char *);