contrib: add a partial-merge tool for sorted lists (such as Python imports) This is a pretty naive tool that uses a regular expression for matching lines. It is based on a Google-internal tool that worked in a similar way. For now, the regular expression is hard-coded to attempt to match single-line Python imports. The only commit I've found in the hg core repo where the tool helped was commit 9cd6292abfdf. I think that's because we often use multiple imports per import statement. I think this tool is still a decent first step (especially once the regex is made configurable in the next patch). The merging should ideally use a proper Python parser and do the merge at the AST (or CST?) level, but that's significantly harder, especially if you want to preserve comments and whitespace. It's also less generic. Differential Revision: https://phab.mercurial-scm.org/D12380