git-pull: dead code removal

Back when a74b1706 (git-pull: disallow implicit merging to detached HEAD,
2007-01-15) added this check, $? referred to the error status of reading
HEAD as a symbolic-ref; but cd67e4d4 (Teach 'git pull' about --rebase,
2007-11-28) moved the command away from where the check is, and nobody
noticed the breakage.  Ever since, $? has always been 0 (tr at the end of
the pipe to find merge_head never fails) and other case arms were never
reached.

These days, error_on_no_merge_candidates function is prepared to handle a
detached HEAD case, which was what the code this patch removes used to
handle.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -182,14 +182,7 @@ merge_head=$(sed -e '/	not-for-merge	/d' \
 
 case "$merge_head" in
 '')
-	case $? in
-	0) error_on_no_merge_candidates "$@";;
-	1) echo >&2 "You are not currently on a branch; you must explicitly"
-	   echo >&2 "specify which branch you wish to merge:"
-	   echo >&2 "  git pull <remote> <branch>"
-	   exit 1;;
-	*) exit $?;;
-	esac
+	error_on_no_merge_candidates "$@"
 	;;
 ?*' '?*)
 	if test -z "$orig_head"