This fixes #97 as it related to grep.

find’s “-name” option never matches a path, so binding grep-find-ignored-files would never allow us to ignore such. Therefore, .projectile paths like the following would not ignore anything when grepping (except for directories unusually bearing extensions):

-/foo/bar.txt

It was only possible to ignore all files in a project with a given name, and only by specifying that name with a leading slash:

-/foo.txt

Specifying a file or directory name without a leading slash would not ignore anything:

-foo.txt
-foo/

Also, if an ignore was a path to a directory that existed from the root, then all such paths to directories in a project would be ignored. However, if the path was to a direct descendant of the root which was not a directory (or did not even exist), then all non-directory files in the project with that direct descendant’s name would be ignored, and if the path was to a 2nd-generation or later non-directory or non-existent descendant, then nothing would be ignored.

These strange cases stemmed from probable misuse of grep-find-ignored-directories, which seems to only be intended for ignoring directory names appearing anywhere, and misuse of grep-find-ignored-files, which seems to only be intended for ignoring file names appearing anywhere and without respect to parent directories. Therefore, binding ignores via grep-find-ignored-directories and grep-find-ignored-files greatly limited what could be ignored.

To ignore paths to files and to get more consistent behavior, I thought it was warranted to write a special exclusion expression supporting paths and which only ignored files and directories relative to the root. Also, to fulfill the promise of the documentation and provide support for relative paths (or patterns as I interpreted it), it was warranted to write another special exclusion expression supporting patterns occurring anywhere.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (make test)
• The new code is not generating bytecode or M-x checkdoc warnings I documented projectile-rgrep-default-command to the same extent as its source function, rgrep-default-command. Let me know if you’d prefer more documentation.
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

Thanks for considering these changes.