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.

cleanup: avoid local vars shadowing imports This will mute some pyflakes "import ... shadowed by loop variable" warnings.

cleanup: avoid _ for local unused tmp variables - that is reserved for i18n _ is usually used for i18n markup but we also used it for I-don't-care variables. Instead, name don't-care variables in a slightly descriptive way but use the _ prefix to designate unused variable. This will mute some pyflakes "import '_' ... shadowed by loop variable" warnings.

cleanup: make sure we always access members of imported modules This will make sure we get import errors, even if demandimport is enabled. This will also mute some pyflakes 'imported but unused' warnings.

largefiles: update lfdirstate for unchanged largefiles during linear merging Before this patch, linear merging of modified largefiles causes an unexpected result, if (1) largefile collides with same-name normal one in the target revision and (2) "local" largefile is chosen, even though branch merging between such revisions works correctly. Expected result of such linear merging is marking the largefile as (re-)"added", but the actual result is marking it as "modified". The standin of modified "local largefile" is not changed by linear merging, and updating/merging update lfdirstate entries only for largefiles of which standins are changed. This patch adds the code path to update lfdirstate only for largefiles of which standins are not changed. In this case, "synclfdirstate" should be invoked with True as "normallookup" argument always to force using "normallookup" on dirstate for "n" files, because "normal" may mark target files as "clean" unexpectedly. To reduce cost of "lfile not in filelist", this patch converts "filelist" to a "set" object: "filelist" is used only in (1) the newly added code path and (2) the next line of "filelist = set(filelist)". This is a temporary way to fix with less changes. For fundamental resolution of this kind of problems in the future, "lfdirstate" should be updated with "dirstate" simultaneously during "merge.update" execution: maybe by hooking "recordupdates" (+ total refactoring around lfdirstate handling)