Add command to find base commit for creating a fixup

docs/Fixup_Commits.md

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+# Fixup Commits
+
+## Background
+
+There's this common scenario that you have a PR in review, the reviewer is
+requesting some changes, and you make those changes and would normally simply
+squash them into the original commit that they came from. If you do that,
+however, there's no way for the reviewer to see what you changed. You could just
+make a separate commit with those changes at the end of the branch, but this is
+not ideal because it results in a git history that is not very clean.
+
+To help with this, git has a concept of fixup commits: you do make a separate
+commit, but the subject of this commit is the string "fixup! " followed by the
+original commit subject. This both tells the reviewer what's going on (you are
+making a change that you later will squash into the designated commit), and it
+provides an easy way to actually perform this squash operation when you are
+ready to do that (before merging).
+
+## Creating fixup commits
+
+You could of course create fixup commits manually by typing in the commit
+message with the prefix yourself. But lazygit has an easier way to do that:
+in the Commits view, select the commit that you want to create a fixup for, and
+press shift-F (for "Create fixup commit for this commit"). This automatically
+creates a commit with the appropriate subject line.
+
+Don't confuse this with the lowercase "f" command ("Fixup commit"); that one
+squashes the selected commit into its parent, this is not what we want here.
+
+## Squashing fixup commits
+
+When you're ready to merge the branch and want to squash all these fixup commits
+that you created, that's very easy to do: select the first commit of your branch
+and hit shift-S (for "Squash all 'fixup!' commits above selected commit
+(autosquash)"). Boom, done.
+
+## Finding the commit to create a fixup for
+
+When you are making changes to code that you changed earlier in a long branch,
+it can be tedious to find the commit to squash it into. Lazygit has a command to
+help you with this, too: in the Files view, press ctrl-f to select the right
+base commit in the Commits view automatically. From there, you can either press
+shift-F to create a fixup commit for it, or shift-A to amend your changes into
+the commit if you haven't published your branch yet.
+
+This command works in many cases, and when it does it almost feels like magic,
+but it's important to understand its limitations because it doesn't always work.
+The way it works is that it looks at the deleted lines of your current
+modifications, blames them to find out which commit those lines come from, and
+if they all come from the same commit, it selects it. So here are cases where it
+doesn't work:
+
+- Your current diff has only added lines, but no deleted lines. In this case
+  there's no way for lazygit to know which commit you want to add them to.
+- The deleted lines belong to multiple different commits. In this case you can
+  help lazygit by staging a set of files or hunks that all belong to the same
+  commit; if some changes are staged, the ctrl-f command works only on those.
+- The found commit is already on master; in this case, lazygit refuses to select
+  it, because it doesn't make sense to create fixups for it, let alone amend to
+  it.
+
+To sum it up: the command works great if you are changing code again that you
+changed or added earlier in the same branch. This is a common enough case to
+make the command useful.