pathspec: stop --*-pathspecs impact on internal parse_pathspec() uses

Normally parse_pathspec() is used on command line arguments where it
can do fancy thing like parsing magic on each argument or adding magic
for all pathspecs based on --*-pathspecs options.

There's another use of parse_pathspec(), where pathspec is needed, but
the input is known to be pure paths. In this case we usually don't
want --*-pathspecs to interfere. And we definitely do not want to
parse magic in these paths, regardless of --literal-pathspecs.

Add new flag PATHSPEC_LITERAL_PATH for this purpose. When it's set,
--*-pathspecs are ignored, no magic is parsed. And if the caller
allows PATHSPEC_LITERAL (i.e. the next calls can take literal magic),
then PATHSPEC_LITERAL will be set.

This fixes cases where git chokes when GIT_*_PATHSPECS are set because
parse_pathspec() indicates it won't take any magic. But
GIT_*_PATHSPECS add them anyway. These are

   export GIT_LITERAL_PATHSPECS=1
   git blame -- something
   git log --follow something
   git log --merge

"git ls-files --with-tree=path" (aka parse_pathspec() in
overlay_tree_on_cache()) is safe because the input is empty, and
producing one pathspec due to PATHSPEC_PREFER_CWD does not take any
magic into account.

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

pathspec.h

@@ -58,6 +58,13 @@ struct pathspec {
 #define PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE (1<<5)
 #define PATHSPEC_PREFIX_ORIGIN (1<<6)
 #define PATHSPEC_KEEP_ORDER (1<<7)
+/*
+ * For the callers that just need pure paths from somewhere else, not
+ * from command line. Global --*-pathspecs options are ignored. No
+ * magic is parsed in each pathspec either. If PATHSPEC_LITERAL is
+ * allowed, then it will automatically set for every pathspec.
+ */
+#define PATHSPEC_LITERAL_PATH (1<<8)
 
 extern void parse_pathspec(struct pathspec *pathspec,
 			   unsigned magic_mask,