diff in pager mode doesn't handle unchanged line that starts with a hyphen

[jarrodldavis]

When viewing a diff in pager mode, tig appears to misread an unchanged line that starts with a hyphen (-) and interpret it as a deleted line. Here are some screenshots of this commit in pager mode, diff mode, and plain output from git.

tig, pager mode (using git show):

tig, diff mode (using tig show):

plain git output (using git --no-pager show):

Relevant configuration:

• I have core.pager in my git config set to tig
• I emptied my tigrc so that only the default settings were used.

[jarrodldavis]

It looks like this was fixed in #877