filemerge: use 'basic' as the default of '[ui] mergemarkers' for safety

Before this patch, 'detailed' is used as the default of '[ui]
mergemarkers'. This embeds non-ASCII characters in tags, branches,
bookmarks, author and/or commit descriptions into merged files in the
encoding specified by '--encoding' global option, 'HGENCODING' or
other locale setting environment variables.

But, if files to be merged use another encoding, this behavior breaks
consistency of encoding in merged files.

For example, ISO-2022-JP or EUC-JP are sometimes used as the file
encoding for Japanese characters, because of historical and/or
environmental reasons, even though UTF-8 or Shift-JIS are ordinarily
used as the terminal encoding.

This can't be resolved automatically, because Mercurial doesn't aware
encoding of managed files.

This patch uses 'basic' as the default of '[ui] mergemarkers' to avoid
embedding encoding sensitive characters for safety.

This patch puts '[ui] mergemarkers = detailed' into default hgrc file
for tests, to reduce changes for tests in this patch.