Update convert help text
Add Mercurial as a source format, clarify that the include directive triggers the exclusion of all not explicitely included files/dirs and use MAPFILE instead of revmapfile in the text, following the short message convention.
changeset = 'changeset: {rev}:{node|short}\n{branches}{tags}{parents}user: {author}\ndate: {date|date}\nsummary: {desc|firstline}\n\n'
changeset_quiet = '{rev}:{node|short}\n'
changeset_verbose = 'changeset: {rev}:{node|short}\n{branches}{tags}{parents}{manifest}user: {author}\ndate: {date|date}\n{files}{file_adds}{file_dels}{file_copies}description:\n{desc|strip}\n\n\n'
changeset_debug = 'changeset: {rev}:{node}\n{branches}{tags}{parents}{manifest}user: {author}\ndate: {date|date}\n{files}{file_adds}{file_dels}{file_copies}{extras}description:\n{desc|strip}\n\n\n'
start_files = 'files: '
file = ' {file}'
end_files = '\n'
start_file_adds = 'files+: '
file_add = ' {file_add}'
end_file_adds = '\n'
start_file_dels = 'files-: '
file_del = ' {file_del}'
end_file_dels = '\n'
start_file_copies = 'copies: '
file_copy = ' {name} ({source})'
end_file_copies = '\n'
parent = 'parent: {rev}:{node|formatnode}\n'
manifest = 'manifest: {rev}:{node}\n'
branch = 'branch: {branch}\n'
tag = 'tag: {tag}\n'
extra = 'extra: {key}={value|stringescape}\n'