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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>