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: Junio C Hamano <gitster@pobox.com>

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;