git log History Simplification

History Simplification

重现的repository在这里 https://github.com/ChuckTest/git-history-test

具体的讨论，参考这个，https://groups.google.com/g/git-users/c/n8Rj-YBUJEw

A more detailed explanation follows.

Suppose you specified foo as the <paths>. We shall call commits that modify foo !TREESAME, and the rest TREESAME. (In a diff filtered for foo, they look different and equal, respectively.)只有修改了foo，才算是!TREESAME，其他的都算做TREESAME，比如修改了其他文件但是没有修改foo

In the following, we will always refer to the same example history to illustrate the differences between simplification settings. We assume that you are filtering for a file foo in this commit graph:

	  .-A---M---N---O---P---Q
	 /     /   /   /   /   /
	I     B   C   D   E   Y
	 \   /   /   /   /   /
	  `-------------'   X

The horizontal line of history A---Q is taken to be the first parent of each merge. The commits are:

I is the initial commit, in which foo exists with contents “asdf”, and a file quux exists with contents “quux”. Initial commits are compared to an empty tree, so I is !TREESAME.
In A, foo contains just “foo”.
B contains the same change as A. Its merge M is trivial and hence TREESAME to all parents. M有两个parent，分别是A和B，但是相对A和B而言，从文件内容来看，是没有差异的
C does not change foo【这里是提交了一个空的commit】, but its merge N changes it to “foobar”, so it is not TREESAME to any parent.
D sets foo to “baz”. Its merge O combines the strings from N and D to “foobarbaz”; i.e., it is not TREESAME to any parent.
E changes quux to “xyzzy”, and its merge P combines the strings to “quux xyzzy”. P is TREESAME【这里的tree same是说没有修改foo文件】 to O, but not to E.
X is an independent root commit that added a new file side, and Y modified it. Y is TREESAME to X. Its merge Q added side to P, and Q is TREESAME to P, but not to Y.

rev-list walks backwards through history, including or excluding commits based on whether --full-history and/or parent rewriting (via --parents or --children) are used. The following settings are available.

整体效果如图所示

Default mode

Commits are included if they are not TREESAME to any parent (though this can be changed, see --sparse below). If the commit was a merge, and it was TREESAME to one parent, follow only that parent. (Even if there are several TREESAME parents, follow only one of them.) Otherwise, follow all parents.

This results in:

	  .-A---N---O
	 /     /   /
	I---------D

默认的模式下，只看foo的日志的话。会自动过滤掉TREESAME的commit。

通过搜索foo来查看的话，会发现BPQ这三个commit中也包含了foo文件，但是针对foo文件来讲，它们都是TREESAME to parent.

git log --decorate --oneline --graph foo

git log --oneline foo

git log --oneline --graph foo

Note how the rule to only follow the TREESAME parent, if one is available, removed B from consideration entirely. C was considered via N, but is TREESAME. Root commits are compared to an empty tree, so I is !TREESAME.

Parent/child relations are only visible with --parents, but that does not affect the commits selected in default mode, so we have shown the parent lines.

--full-history without parent rewriting 【--graph的作用是，启用parent rewriting】

This mode differs from the default in one point: always follow all parents of a merge, even if it is TREESAME to one of them. Even if more than one side of the merge has commits that are included, this does not imply that the merge itself is! In the example, we get

git log --oneline --full-history foo

	I  A  B  N  D  O  P  Q

M was excluded because it is TREESAME to both parents. E, C and B were all walked, but only B was !TREESAME, so the others do not appear.

Note that without parent rewriting, it is not really possible to talk about the parent/child relationships between the commits, so we show them disconnected.

--full-history with parent rewriting

Ordinary commits are only included if they are !TREESAME (though this can be changed, see --sparse below).

Merges are always included. However, their parent list is rewritten: Along each parent, prune away commits that are not included themselves. This results in

git log --decorate --oneline --graph --full-history foo

	  .-A---M---N---O---P---Q
	 /     /   /   /   /
	I     B   /   D   /
	 \   /   /   /   /
	  `-------------'

Compare to --full-history without rewriting above. Note that E was pruned away because it is TREESAME, but the parent list of P was rewritten to contain E's parent I. The same happened for C and N, and X, Y and Q.