feat: GitDownloader supports cache

[zong-zhe]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• [ ] N
• [x] Y

re #384

2. What is the scope of this PR (e.g. component or file name):

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• [x] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• [x] N
• [ ] Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• [x] Unit test
• [ ] Integration test
• [ ] Benchmark (add benchmark stats below)
• [ ] Manual test (add detailed scripts or steps below)
• [ ] Other

[coveralls]

Pull Request Test Coverage Report for Build 11551360709

Details

• 57 of 109 (52.29%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 41.72%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/git/getter.go	7	9	77.78%
pkg/downloader/source.go	11	16	68.75%
pkg/git/git.go	0	7	0.0%
pkg/downloader/downloader.go	39	77	50.65%
<!--	Total:	57	109	52.29%	-->

Totals
Change from base Build 11477327992:	0.2%
Covered Lines:	3653
Relevant Lines:	8756

---

💛 - Coveralls