diff --abbrev=<n> option fix.
diff --abbrev=<n> option fix.

Earier specifying an abbreviation shorter than minimum fell back
to full 40 letters, which was nonsense.  Make it to fall back to
the minimum number (currently 4).

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git