completion: complete --diff-merges, its options and --no-diff-merges

The flags --[no-]diff-merges only make sense for 'git log' and 'git
show', so add a new variable __git_log_show_options for options only
relevant to these two commands, and add them there. Also add
__git_diff_merges_opts and list the accepted values for --diff-merges,
and use it in _git_log and _git_show.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2040,6 +2040,12 @@ __git_log_shortlog_options="
 	--author= --committer= --grep=
 	--all-match --invert-grep
 "
+# Options accepted by log and show
+__git_log_show_options="
+	--diff-merges --diff-merges= --no-diff-merges
+"
+
+__git_diff_merges_opts="off none on first-parent 1 separate m combined c dense-combined cc remerge r"
 
 __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:"
@@ -2096,11 +2102,16 @@ _git_log ()
 		__gitcomp "sorted unsorted" "" "${cur##--no-walk=}"
 		return
 		;;
+	--diff-merges=*)
+                __gitcomp "$__git_diff_merges_opts" "" "${cur##--diff-merges=}"
+                return
+                ;;
 	--*)
 		__gitcomp "
 			$__git_log_common_options
 			$__git_log_shortlog_options
 			$__git_log_gitk_options
+			$__git_log_show_options
 			--root --topo-order --date-order --reverse
 			--follow --full-diff
 			--abbrev-commit --no-abbrev-commit --abbrev=
@@ -3015,10 +3026,15 @@ _git_show ()
 		__gitcomp "$__git_ws_error_highlight_opts" "" "${cur##--ws-error-highlight=}"
 		return
 		;;
+	--diff-merges=*)
+                __gitcomp "$__git_diff_merges_opts" "" "${cur##--diff-merges=}"
+                return
+                ;;
 	--*)
 		__gitcomp "--pretty= --format= --abbrev-commit --no-abbrev-commit
 			--oneline --show-signature
 			--expand-tabs --expand-tabs= --no-expand-tabs
+			$__git_log_show_options
 			$__git_diff_common_options
 			"
 		return