diff --no-index: support limiting by pathspec

The --no-index option of git-diff enables using the diff machinery from git while operating outside of a repository. This mode of git diff is able to compare directories and produce a diff of their contents. When operating git diff in a repository, git has the notion of "pathspecs" which can specify which files to compare. In particular, when using git to diff two trees, you might invoke: $ git diff-tree -r <treeish1> <treeish2>. where the treeish could point to a subdirectory of the repository. When invoked this way, users can limit the selected paths of the tree by using a pathspec. Either by providing some list of paths to accept, or by removing paths via a negative refspec. The git diff --no-index mode does not support pathspecs, and cannot limit the diff output in this way. Other diff programs such as GNU difftools have options for excluding paths based on a pattern match. However, using git diff as a diff replacement has several advantages over many popular diff tools, including coloring moved lines, rename detections, and similar. Teach git diff --no-index how to handle pathspecs to limit the comparisons. This will only be supported if both provided paths are directories. For comparisons where one path isn't a directory, the --no-index mode already has some DWIM shortcuts implemented in the fixup_paths() function. Modify the fixup_paths function to return 1 if both paths are directories. If this is the case, interpret any extra arguments to git diff as pathspecs via parse_pathspec. Use parse_pathspec to load the remaining arguments (if any) to git diff --no-index as pathspec items. Disable PATHSPEC_ATTR support since we do not have a repository to do attribute lookup. Disable PATHSPEC_FROMTOP since we do not have a repository root. All pathspecs are treated as rooted at the provided comparison paths. After loading the pathspec data, calculate skip offsets for skipping past the root portion of the paths. This is required to ensure that pathspecs start matching from the provided path, rather than matching from the absolute path. We could instead pass the paths as prefix values to parse_pathspec. This is slightly problematic because the paths come from the command line and don't necessarily have the proper trailing slash. Additionally, that would require parsing pathspecs multiple times. Pass the pathspec object and the skip offsets into queue_diff, which in-turn must pass them along to read_directory_contents. Modify read_directory_contents to check against the pathspecs when scanning the directory. Use the skip offset to skip past the initial root of the path, and only match against portions that are below the intended directory structure being compared. The search algorithm for finding paths is recursive with read_dir. To make pathspec matching work properly, we must set both DO_MATCH_DIRECTORY and DO_MATCH_LEADING_PATHSPEC. Without DO_MATCH_DIRECTORY, paths like "a/b/c/d" will not match against pathspecs like "a/b/c". This is usually achieved by setting the is_dir parameter of match_pathspec. Without DO_MATCH_LEADING_PATHSPEC, paths like "a/b/c" would not match against pathspecs like "a/b/c/d". This is crucial because we recursively iterate down the directories. We could simply avoid checking pathspecs at subdirectories, but this would force recursion down directories which would simply be skipped. If we always passed DO_MATCH_LEADING_PATHSPEC, then we will incorrectly match in certain cases such as matching 'a/c' against ':(glob)**/d'. The match logic will see that a matches the leading part of the **/ and accept this even tho c doesn't match. To avoid this, use the match_leading_pathspec() variant recently introduced. This sets both flags when is_dir is set, but leaves them both cleared when is_dir is 0. Add test cases and documentation covering the new functionality. Note for the documentation I opted not to move the placement of '--' which is sometimes used to disambiguate arguments. The diff --no-index mode requires exactly 2 arguments determining what to compare. Any additional arguments are interpreted as pathspecs and must come afterwards. Use of '--' would not actually disambiguate anything, since there will never be ambiguity over which arguments represent paths or pathspecs. Signed-off-by: Jacob Keller <jacob.keller@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>