completion: add comments describing __git_diff_* globals

Add descriptive comments for '__git_diff_common_options' and
'__git_diff_difftool_options', so that it is clearer when looking at
these variables to know in which command's completion they are used.

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

contrib/completion/git-completion.bash

@@ -1733,6 +1733,7 @@ __git_color_moved_opts="no default plain blocks zebra dimmed-zebra"
 __git_color_moved_ws_opts="no ignore-space-at-eol ignore-space-change
 			ignore-all-space allow-indentation-change"
 
+# Options for the diff machinery (diff, log, show, stash, range-diff, ...)
 __git_diff_common_options="--stat --numstat --shortstat --summary
 			--patch-with-stat --name-only --name-status --color
 			--no-color --color-words --no-renames --check
@@ -1757,6 +1758,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--anchored=
 "
 
+# Options for diff/difftool
 __git_diff_difftool_options="--cached --staged --pickaxe-all --pickaxe-regex
 			--base --ours --theirs --no-index --relative --merge-base
 			$__git_diff_common_options"