revert: drop `missingmodified` set There cannot be any elements in this set since: dsmodified &= modified

revert: detect files added during a merge In case of merge, file that are actually added can be reported as modified. This is currently handled by special-case code. We detect it beforehand instead. This will lets use remove the special-case code at some point in the future.

revert: simplify handling of `added` files There are multiple possible cases for added files. But it's all handled by magic much lower in the stack. We document them, simplify the codes and move on.

merge with stable

rebase: use "rebase.collapse" as "editform" for "--collapse" always Before this patch, if both "--message" and "--collapse" are specified for "hg rebase", "rebaes.normal" is used as "editform" unexpectedly. Unlike patches before and after in this series for improvement, this is bug fix patch.

ui: invoke editor for committing with HGEDITFORM environment variable At the external editor invocation for committing, the value specified as "editform" for "cmdutil.getcommiteditor" is in "HGEDITFORM". This enables external editor to do own customization according to commit types.

cleanup: name unused variables using convention of leading _ This helps checker tools ... and readability for those who knows and follows the convention.

cleanup: rename check-translation.py checker function - don't hide global var

cleanup: remove some unused / duplicate imports

cleanup: fix some list comprehension redefinitions of existing vars In all the remaining cases the comprehension variable is used for the same thing as a previous loop variable. This will mute some pyflakes "list comprehension redefines" warnings.