rhg: Don’t compare ambiguous files one byte at a time Even though the use of `BufReader` reduces the number of syscalls to read the file from disk, `.bytes()` yields a separate `Result` for every byte. Creating those results and dispatching on them is most likely costly. Instead, this commit opts for simplicity by reading the entire file into memory and comparing a single pair of byte strings. Note that memory already needs to contain the entire previous contents of the file, as read from the filelog. So with an extremely large file this doubles memory use but does not make it grow by orders of magnitude. At first I wrote code that still avoids reading the entire file into memory and compares one buffer at a time with `BufReader`. Find this code below for posterity. However its correctness is subtle. I ended up preferring the simplicity of the obviously-correct single comparison. ```rust let mut reader = BufReader::new(fobj); let mut expected = &contents_in_p1[..]; loop { let buf = reader.fill_buf().when_reading_file(&fs_path)?; if buf.is_empty() { // Found EOF return Ok(expected.is_empty()); } else if let Some(rest) = expected.drop_prefix(buf) { // What we read so far matches the expected content, continue reading let buf_len = buf.len(); reader.consume(buf_len); expected = rest } else { // Found different content return Ok(false); } } ``` Differential Revision: https://phab.mercurial-scm.org/D11412

rhg: Reuse manifest when checking status of multiple ambiguous files When `rhg status` cannot determine whether a file is clean based on mtime and size alone, it needs to compare its contents with those found in the parent commit. Previously, rhg would find the (same) manifest of that commit again for every such file. This is lifted out of the loop and reused. Differential Revision: https://phab.mercurial-scm.org/D11411

rust: Return HgError instead of RevlogError in revlog constructors This leaves fewer cases for callers to handle, as RevlogError is more general Differential Revision: https://phab.mercurial-scm.org/D11410

rhg: Align with Python on some revset parsing corner cases In particular: * A string of ASCII digits can be either an integer on a hex prefix * The NULL node ID should convert to the NULL revision number Differential Revision: https://phab.mercurial-scm.org/D11409

rust: Add a Filelog struct that wraps Revlog Some filelog-specific logic is moved from code `rhg cat` into this struct where it can better be reused. Additionally, a missing end delimiter for metadata causes an error to be returned instead of being silently ignored. Differential Revision: https://phab.mercurial-scm.org/D11408

rust: Add Repo::manifest(revision) This deduplicates some common code. Differential Revision: https://phab.mercurial-scm.org/D11407

rust: Keep lazily-initialized Changelog and Manifest log on the Repo object That way if one of them is accessed multiple times it won’t be reopened from the filesystem. Differential Revision: https://phab.mercurial-scm.org/D11406

rust: Move lazy initialization of `Repo::dirstate_map` into a generic struct More components of `Repo` will be added following the same pattern. Differential Revision: https://phab.mercurial-scm.org/D11405

rust: Rename Manifest to Manifestlog, ManifestEntry to Manifest This appears to match the terminology used in Python code and on https://www.mercurial-scm.org/wiki/Manifest Differential Revision: https://phab.mercurial-scm.org/D11404

rust: Add Repo::dirstate_map and use it in `rhg status` This moves low-level dirstate wrangling out of the status command and into a more reusable location. The open dirstate map is lazily initialized and kept on the Repo object, for reuse by sub-sequent calls. Differential Revision: https://phab.mercurial-scm.org/D11398