tests/test-issue1877.t

match: introduce boolean prefix() method

tl;dr: This is another step towards a (previously unstated) goal of
eliminating match.files() in conditions.

There are four types of matchers:

 * always: Matches everything, checked with always(), files() is empty

 * exact: Matches exact set of files, checked with isexact(), files()
   contains the files to match

 * patterns: Matches more complex patterns, checked with anypats(),
   files() contains roots of the matched patterns

 * prefix: Matches simple 'path:' patterns as prefixes ('foo' matches
   both 'foo' and 'foo/bar'), no single method to check, files()
   contains the prefixes to match

For completeness, it would be nice to have a method for checking for
the "prefix" type of matcher as well, so let's add that, making it
return True simply when none of the others do.

The larger goal here is to eliminate uses of match.files() in
conditions (i.e. bool(match.files())). The reason for this is that
there are scenarios when you would like to create a "prefix" matcher
that happens to match no files. One example is for 'hg files -I foo
bar'. The narrowmatcher also restricts the set of files given and it
would not surprise me if have bugs caused by that already. Note that
'if m.files() and not m.anypats()' and similar is sometimes used to
catch the "exact" and "prefix" cases above.

http://mercurial.selenic.com/bts/issue1877

  $ hg init a
  $ cd a
  $ echo a > a
  $ hg add a
  $ hg ci -m 'a'
  $ echo b > a
  $ hg ci -m'b'
  $ hg up 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg book main
  $ hg book
   * main                      0:cb9a9f314b8b
  $ echo c > c
  $ hg add c
  $ hg ci -m'c'
  created new head
  $ hg book
   * main                      2:d36c0562f908
  $ hg heads
  changeset:   2:d36c0562f908
  bookmark:    main
  tag:         tip
  parent:      0:cb9a9f314b8b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     c
  
  changeset:   1:1e6c11564562
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     b
  
  $ hg up 1e6c11564562
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (leaving bookmark main)
  $ hg merge main
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg book
     main                      2:d36c0562f908
  $ hg ci -m'merge'
  $ hg book
     main                      2:d36c0562f908

  $ cd ..