Improve git repository performance

[Jun Omae]

We often use log: links with git repository in ticket. The ticket view with the links is too slow. We should improve the performance of git repository.

1. The git connector constructs revisions cache for parents, children, branches and tags. The construction is performed each request if persistent_cache option is disabled. We could improve it.
2. The revisions cache is created each request if repository is not modified. Caching refs using git for-each-ref and detecting the modification could reduce times of the creation.
3. git for-each-ref is executed to the number of shown changesets in log view. Caching name and revision of tags could avoid the executions.

[Jun Omae]

1. The git connector constructs revisions cache for parents, children, branches and tags. The construction is performed each request if persistent_cache option is disabled. We could improve it.

Especially, that's pretty slow if a repository has many branches.

Proposed changes in [4c265be70/jomae.git].

	before	after
jomae.git	3077.2ms	291.9ms
libgit2	1182.7ms	393.4ms

[Jun Omae]

3. git for-each-ref is executed to the number of shown changesets in log view. Caching name and revision of tags could avoid the executions.

Proposed changes in [bcc519c7c/jomae.git].

Timing of log view with the following repository.

	before	after
jomae.git	2709ms	1097ms
libgit2	2755ms	1248ms

[Jun Omae]

2. The revisions cache is created each request if repository is not modified. Caching refs using git for-each-ref and detecting the modification could reduce times of the creation.

In [03c1d875c/jomae.git], detects that git repository is modified using git show-ref and creates revisions cache only when it is modified.

[Jun Omae]

Committed in [13951] and merged to trunk in [13952].

I noticed changeset view retrieves changes of the changeset twice. In [13953], reduced to once.