record: when backing up, avoid generating very long filenames If the original file's path is longer than the individual filename maximum length (256 on Linux, I believe?), then this mechanism of "replace slashes with underscores" causes an error. Now, we'll produce just the "basename" of the file, plus some stuff to ensure it's unique. This can be potentially confusing for users if there's a file with the same name in multiple directories, but I suspect that this is better than just breaking. Example: `<reporoot>/a/long/path/to/somefile.txt` used to be backed up as `<reporoot>/.hg/record-backups/a_long_path_to_somefile.txt.abcdefgh`, it will now be backed up as `<reporoot>/.hg/record-backups/somefile.txt.abcdefgh` We could do the naive thing (what we were doing before) and have it to doing something with either subdirectories (`<backuproot>/a/long/path/to/somefile.txt.abcdefgh` or minimize #dirs with `<backuproot>/a_long_path/to_somefile.txt.abcdefgh`), prefix-truncated paths (such as `<backuproot>/__ath_to_somefile.txt.abcdefgh`, where that `__` elides enough to get us under 255 chars (counting the +9 we need to add!)), or hash-of-dirname (`<backuproot>/<sha1sum_of_dirname>/somefile.txt.abcdefgh`), but ultimately every option felt over engineered and that it would be more likely to cause problems than it would be to solve any, especially if it was conditional on directory length. Differential Revision: https://phab.mercurial-scm.org/D9207