tests/test-symlink-root.out

merge: allow smarter tool configuration

Add [merge-tool] hgrc section with:
 <tool>.executable = name or path (<tool>)
 <tool>.args = args with $local/base/other/output ($local $base $other)
 <tool>.priority = priority (default 0)
 <tool>.binary = handles binary (False)
 <tool>.symlink = handles symlinks (False)
 <tool>.checkconflict = check for conflict markers (False)
 <tool>.premerge = try internal simplemerge (True if not binary or symlink)

Four built-in tools: internal:{merge,local,other,fail}

Add [merge-patterns] section of the form:
 <pattern> = <tool>

Priority of settings is:
 HGMERGE
 merge-patterns
 ui:merge
 merge-tools by priority
 hgmerge, if it can be found

Changes:
 unsuccessful merges leave .orig files

? foo
? foo