tests/test-merge4.t

help: eliminate duplicate text for revset string patterns

There's no reason to duplicate this so many times, and it's likely an instance
will be missed if support for a new pattern is added and documented.  The
stringmatcher is mostly used by revsets, though it is also used for the 'tag'
related templates, and namespace filtering in the journal extension.  So maybe
there's a better place to document it.  `hg help patterns` seems inappropriate,
because that is all file pattern matching.

While here, indicate how to perform case insensitive regex searches.

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg update 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo This is file c1 > c
  $ hg add c
  $ hg commit -m "commit #2"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ rm b
  $ echo This is file c22 > c

Test hg behaves when committing with a missing file added by a merge

  $ hg commit -m "commit #3"
  abort: cannot commit merge with missing files
  [255]