completion: offer ctags symbol names for 'git log -S', '-G' and '-L:'

Just like in the case of search patterns for 'git grep', see 29eec71f2
(completion: match ctags symbol names in grep patterns, 2011-10-21)),
a common thing to look for using 'git log -S', '-G' and '-L:' is the
name of a symbol.

Teach the completion for 'git log' to offer ctags symbol names after
these options, both in stuck and in unstuck forms.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1687,6 +1687,19 @@ _git_log ()
 	if [ -f "$__git_repo_path/MERGE_HEAD" ]; then
 		merge="--merge"
 	fi
+	case "$prev,$cur" in
+	-L,:*:*)
+		return	# fall back to Bash filename completion
+		;;
+	-L,:*)
+		__git_complete_symbol --cur="${cur#:}" --sfx=":"
+		return
+		;;
+	-G,*|-S,*)
+		__git_complete_symbol
+		return
+		;;
+	esac
 	case "$cur" in
 	--pretty=*|--format=*)
 		__gitcomp "$__git_log_pretty_formats $(__git_pretty_aliases)
@@ -1732,6 +1745,21 @@ _git_log ()
 			"
 		return
 		;;
+	-L:*:*)
+		return	# fall back to Bash filename completion
+		;;
+	-L:*)
+		__git_complete_symbol --cur="${cur#-L:}" --sfx=":"
+		return
+		;;
+	-G*)
+		__git_complete_symbol --pfx="-G" --cur="${cur#-G}"
+		return
+		;;
+	-S*)
+		__git_complete_symbol --pfx="-S" --cur="${cur#-S}"
+		return
+		;;
 	esac
 	__git_complete_revlist
 }