filemerge: remove some redundancy in decorators/docstrings

filemerge: create detail of internal merge tools from documentation string this patch introduces 'internaltoolsmarker' which creates detail of each internal merge tools from documentation string for 'hg help merge-tools'.

filemerge: refactoring of 'filemerge()' current 'filemerge.filemerge()' implementation is verfy complicated. - it is not easy to add new internal merge tools (only by patching on 'filemerge()', or replacing it completely) - cleanup of temporary files is unsatisfactory ('internal:dump' does not, in fact) this is patch for refactoring of 'filemerge()' to isolate each internal merge tool implementations from 'filemerge()', and clean up common part in it.

patch: fuzz more aggressively to match patch(1) behaviour The previous code was assuming a default context of 3 lines. When fuzzing, it would take this value in account to reduce the amount of removed line from hunks top or bottom. For instance, if a hunk has only 2 lines of bottom context, fuzzing with fuzz=1 would do nothing and with fuzz=2 it would remove one of those lines. A hunk with one line of bottom context could not be fuzzed at all. patch(1) has apparently no such restrictions and takes the fuzz level at face value. - test-import.t: fuzz/offset changes at the beginning of file are explained by the new fuzzing behaviour and match patch(1) ones. Patching locations are different but those of my patch(1) do not make a lot of sense right now (patched output are the same) - test-import-bypass.t: more agressive fuzzing makes a patching supposed to fail because of context, succeed. Change the diff to avoid this. - test-mq-merge.t: more agressive fuzzing would allow the merged patch to apply with fuzz, but fortunately we disallow this behaviour. The new output is kept. I have not enough experience with patch(1) fuzzing to know whether aligning our implementation on it is a good or bad idea. Until now, it has been the implementation reference. For instance, "qpush" tolerates fuzz (test-mq-merge.t runs the special case of pushing merge revisions where fuzzing is forbidden).