completion: new function __git_complete_log_opts

The options accepted by git-log are also accepted by at least one other
command (git-bisect).  Factor the common option completion code into a
new function and use it from _git_log.  The new function leaves
COMPREPLY empty if no option candidates are found, so that callers can
safely check it to determine if completion for other arguments should be
attempted.

Signed-off-by: Britton Leo Kerin <britton.kerin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2089,10 +2089,12 @@ __git_diff_merges_opts="off none on first-parent 1 separate m combined c dense-c
 __git_log_pretty_formats="oneline short medium full fuller reference email raw format: tformat: mboxrd"
 __git_log_date_formats="relative iso8601 iso8601-strict rfc2822 short local default human raw unix auto: format:"
 
-_git_log ()
+# Complete porcelain (i.e. not git-rev-list) options and at least some
+# option arguments accepted by git-log.  Note that this same set of options
+# are also accepted by some other git commands besides git-log.
+__git_complete_log_opts ()
 {
-	__git_has_doubledash && return
-	__git_find_repo_path
+	COMPREPLY=()
 
 	local merge=""
 	if [ -f "$__git_repo_path/MERGE_HEAD" ]; then
@@ -2186,6 +2188,16 @@ _git_log ()
 		return
 		;;
 	esac
+}
+
+_git_log ()
+{
+	__git_has_doubledash && return
+	__git_find_repo_path
+
+	__git_complete_log_opts
+        [ ${#COMPREPLY[@]} -eq 0 ] || return
+
 	__git_complete_revlist
 }