Support dependency relations between patch series

從 git pw patch apply 和 git pw series apply 入口點開始，並檢視相關 issue #71 以了解現有的脈絡。完成的標準是：依列出的順序收集補丁或 cover letter 中的 Depends-on 標籤，將其轉換為系列間相依性，並在不進行遞迴走訪的情況下自動套用。

The DPDK project has defined syntax for a "Depends-on" tag. A contributor can add this label to their patch or cover letter if it depends on another patch or series that has not yet been applied to the target branch.

The syntax for this tag and where it appears in a patch file is as follows:

... MIME headers and such go here ...

Some commit message

Signed-off-by: someone
---
Depends-on: patch-##### ("title of patch")
Depends-on: series-##### ("title of series")

... git diff summary goes here ...

---
... diff goes here ...

These tags may also appear in a cover letter for a series.

There can be multiple of these tags defined with differing values. An implementation is intended to collect them into a list. When applying a patch which has these tags, they should be applied in the order listed in the patch file. This process is not recursive, as in if the dependents of a patch have additional dependents, those are not applied.

All dependencies are translated to inter-series dependencies. As in, if a patch a in series A references a patch b in series B, then we can infer that series A depends on series B. Same thing if a patch a depends on series B.

The ask is for, when running git pw patch apply or git pw series apply , the command will scan for these dependent patches or series and apply them automatically as described above.

Related: https://github.com/getpatchwork/git-pw/issues/71