setup: don't dereference in-tree symlinks for absolute paths

The prefix_path_gently() function currently applies real_path to
everything if given an absolute path, dereferencing symlinks both
outside and inside the work tree.

This causes most high-level functions to misbehave when acting on
symlinks given via absolute paths. For example

	$ git add /dir/repo/symlink

attempts to add the target of the symlink rather than the symlink
itself, which is usually not what the user intends to do.

In order to manipulate symlinks in the work tree using absolute paths,
symlinks should only be dereferenced outside the work tree.

Modify the prefix_path_gently() to first normalize the path in order to
make sure path levels are separated by '/', then pass the result to
'abspath_part_inside_repo' to find the part inside the work tree
(without dereferencing any symlinks inside the work tree).

For absolute paths, prefix_path_gently() did not, nor does now do, any
actual prefixing, hence the result from abspath_part_in_repo() is
returned as-is.

Fixes t0060-82 and t3004-5.

Signed-off-by: Martin Erik Werner <martinerikwerner@gmail.com>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3004-ls-files-basic.sh

@@ -36,7 +36,7 @@ test_expect_success 'ls-files -h in corrupt repository' '
 	test_i18ngrep "[Uu]sage: git ls-files " broken/usage
 '
 
-test_expect_failure SYMLINKS 'ls-files with absolute paths to symlinks' '
+test_expect_success SYMLINKS 'ls-files with absolute paths to symlinks' '
 	mkdir subs &&
 	ln -s nosuch link &&
 	ln -s ../nosuch subs/link &&