tag, branch, for-each-ref: add --ignore-case for sorting and filtering

This options makes sorting ignore case, which is great when you have
branches named bug-12-do-something, Bug-12-do-some-more and
BUG-12-do-what and want to group them together. Sorting externally may
not be an option because we lose coloring and column layout from
git-branch and git-tag.

The same could be said for filtering, but it's probably less important
because you can always go with the ugly pattern [bB][uU][gG]-* if you're
desperate.

You can't have case-sensitive filtering and case-insensitive sorting (or
the other way around) with this though. For branch and tag, that should
be no problem. for-each-ref, as a plumbing, might want finer control.
But we can always add --{filter,sort}-ignore-case when there is a need
for it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -118,6 +118,10 @@ OPTIONS
 	default to color output.
 	Same as `--color=never`.
 
+-i::
+--ignore-case::
+	Sorting and filtering branches are case insensitive.
+
 --column[=<options>]::
 --no-column::
 	Display branch listing in columns. See configuration variable

Documentation/git-for-each-ref.txt

@@ -79,6 +79,9 @@ OPTIONS
 	Only list refs which contain the specified commit (HEAD if not
 	specified).
 
+--ignore-case::
+	Sorting and filtering refs are case insensitive.
+
 FIELD NAMES
 -----------
 

Documentation/git-tag.txt

@@ -108,6 +108,10 @@ OPTIONS
 	variable if it exists, or lexicographic order otherwise. See
 	linkgit:git-config[1].
 
+-i::
+--ignore-case::
+	Sorting and filtering tags are case insensitive.
+
 --column[=<options>]::
 --no-column::
 	Display tag listing in columns. See configuration variable